add MailKit and MimeKit

src/MailKit/AccessControl.cs

@@ -0,0 +1,137 @@
+//
+// AccessControl.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Collections.Generic;
+
+namespace MailKit {
+	/// <summary>
+	/// An Access Control.
+	/// </summary>
+	/// <remarks>
+	/// An Access Control is a set of permissions available for a particular identity,
+	/// controlling whether or not that identity has the ability to perform various tasks.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+	/// </example>
+	public class AccessControl
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AccessControl"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.AccessControl"/> with the given name and
+		/// access rights.
+		/// </remarks>
+		/// <param name="name">The identifier name.</param>
+		/// <param name="rights">The access rights.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="name"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="rights"/> is <c>null</c>.</para>
+		/// </exception>
+		public AccessControl (string name, IEnumerable<AccessRight> rights)
+		{
+			if (name == null)
+				throw new ArgumentNullException (nameof (name));
+
+			Rights = new AccessRights (rights);
+			Name = name;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AccessControl"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.AccessControl"/> with the given name and
+		/// access rights.
+		/// </remarks>
+		/// <param name="name">The identifier name.</param>
+		/// <param name="rights">The access rights.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="name"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="rights"/> is <c>null</c>.</para>
+		/// </exception>
+		public AccessControl (string name, string rights)
+		{
+			if (name == null)
+				throw new ArgumentNullException (nameof (name));
+
+			Rights = new AccessRights (rights);
+			Name = name;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AccessControl"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.AccessControl"/> with the given name and no
+		/// access rights.
+		/// </remarks>
+		/// <param name="name">The identifier name.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="name"/> is <c>null</c>.
+		/// </exception>
+		public AccessControl (string name)
+		{
+			if (name == null)
+				throw new ArgumentNullException (nameof (name));
+
+			Rights = new AccessRights ();
+			Name = name;
+		}
+
+		/// <summary>
+		/// The identifier name for the access control.
+		/// </summary>
+		/// <remarks>
+		/// The identifier name for the access control.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The identifier name.</value>
+		public string Name {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the access rights.
+		/// </summary>
+		/// <remarks>
+		/// Gets the access rights.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The access rights.</value>
+		public AccessRights Rights {
+			get; private set;
+		}
+	}
+}

src/MailKit/AccessControlList.cs

@@ -0,0 +1,66 @@
+//
+// AccessControlList.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Collections.Generic;
+
+namespace MailKit {
+	/// <summary>
+	/// An Access Control List (ACL)
+	/// </summary>
+	/// <remarks>
+	/// An Access Control List (ACL) is a list of access controls defining the permissions
+	/// various identities have available.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+	/// </example>
+	public class AccessControlList : List<AccessControl>
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AccessControlList"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.AccessControlList"/>.
+		/// </remarks>
+		/// <param name="controls">The list of access controls.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="controls"/> is <c>null</c>.
+		/// </exception>
+		public AccessControlList (IEnumerable<AccessControl> controls) : base (controls)
+		{
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AccessControlList"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.AccessControlList"/>.
+		/// </remarks>
+		public AccessControlList ()
+		{
+		}
+	}
+}

src/MailKit/AccessRight.cs

@@ -0,0 +1,232 @@
+//
+// AccessRight.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// An individual Access Right to be used with ACLs.
+	/// </summary>
+	/// <remarks>
+	/// <para>An individual Access Right meant to be used with
+	/// <see cref="AccessControlList"/>.</para>
+	/// <para>For more information on what rights are available,
+	/// see https://tools.ietf.org/html/rfc4314#section-2.1
+	/// </para>
+	/// </remarks>
+	public struct AccessRight : IEquatable<AccessRight>
+	{
+		/// <summary>
+		/// The access right for folder lookups.
+		/// </summary>
+		/// <remarks>
+		/// Allows the <see cref="MailKit.IMailFolder"/> to be visible when listing folders.
+		/// </remarks>
+		public static readonly AccessRight LookupFolder = new AccessRight ('l');
+
+		/// <summary>
+		/// The access right for opening a folder and getting the status.
+		/// </summary>
+		/// <remarks>
+		/// Provides access for opening and getting the status of the folder.
+		/// </remarks>
+		public static readonly AccessRight OpenFolder = new AccessRight ('r');
+
+		/// <summary>
+		/// The access right for adding or removing the Seen flag on messages in the folder.
+		/// </summary>
+		/// <remarks>
+		/// Provides access to add or remove the <see cref="MessageFlags.Seen"/> flag on messages within the
+		/// <see cref="MailKit.IMailFolder"/>.
+		/// </remarks>
+		public static readonly AccessRight SetMessageSeen = new AccessRight ('s');
+
+		/// <summary>
+		/// The access right for adding or removing flags (other than Seen and Deleted)
+		/// on messages in a folder.
+		/// </summary>
+		/// <remarks>
+		/// Provides access to add or remove the <see cref="MessageFlags"/> on messages
+		/// (other than <see cref="MessageFlags.Seen"/> and
+		/// <see cref="MessageFlags.Deleted"/>) within the folder.
+		/// </remarks>
+		public static readonly AccessRight SetMessageFlags = new AccessRight ('w');
+
+		/// <summary>
+		/// The access right allowing messages to be appended or copied into the folder.
+		/// </summary>
+		/// <remarks>
+		/// Provides access to append or copy messages into the folder.
+		/// </remarks>
+		public static readonly AccessRight AppendMessages = new AccessRight ('i');
+
+		/// <summary>
+		/// The access right allowing subfolders to be created.
+		/// </summary>
+		/// <remarks>
+		/// Provides access to create subfolders.
+		/// </remarks>
+		public static readonly AccessRight CreateFolder = new AccessRight ('k');
+
+		/// <summary>
+		/// The access right for deleting a folder and/or its subfolders.
+		/// </summary>
+		/// <remarks>
+		/// Provides access to delete the folder and/or any subfolders.
+		/// </remarks>
+		public static readonly AccessRight DeleteFolder = new AccessRight ('x');
+
+		/// <summary>
+		/// The access right for adding or removing the Deleted flag to messages within a folder.
+		/// </summary>
+		/// <remarks>
+		/// Provides access to add or remove the <see cref="MessageFlags.Deleted"/> flag from
+		/// messages within the folder. It also provides access for setting the
+		/// <see cref="MessageFlags.Deleted"/> flag when appending a message to a folder.
+		/// </remarks>
+		public static readonly AccessRight SetMessageDeleted = new AccessRight ('t');
+
+		/// <summary>
+		/// The access right for expunging deleted messages in a folder.
+		/// </summary>
+		/// <remarks>
+		/// Provides access to expunge deleted messages in a folder.
+		/// </remarks>
+		public static readonly AccessRight ExpungeFolder = new AccessRight ('e');
+
+		/// <summary>
+		/// The access right for administering the ACLs of a folder.
+		/// </summary>
+		/// <remarks>
+		/// Provides administrative access to change the ACLs for the folder.
+		/// </remarks>
+		public static readonly AccessRight Administer = new AccessRight ('a');
+
+		/// <summary>
+		/// The character representing the particular access right.
+		/// </summary>
+		/// <remarks>
+		/// Represents the character value of the access right.
+		/// </remarks>
+		public readonly char Right;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AccessRight"/> struct.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.AccessRight"/> struct.
+		/// </remarks>
+		/// <param name="right">The access right.</param>
+		public AccessRight (char right)
+		{
+			Right = right;
+		}
+
+		#region IEquatable implementation
+
+		/// <summary>
+		/// Determines whether the specified <see cref="MailKit.AccessRight"/> is equal to the current <see cref="MailKit.AccessRight"/>.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether the specified <see cref="MailKit.AccessRight"/> is equal to the current <see cref="MailKit.AccessRight"/>.
+		/// </remarks>
+		/// <param name="other">The <see cref="MailKit.AccessRight"/> to compare with the current <see cref="MailKit.AccessRight"/>.</param>
+		/// <returns><c>true</c> if the specified <see cref="MailKit.AccessRight"/> is equal to the current
+		/// <see cref="MailKit.AccessRight"/>; otherwise, <c>false</c>.</returns>
+		public bool Equals (AccessRight other)
+		{
+			return other.Right == Right;
+		}
+
+		#endregion
+
+		/// <summary>
+		/// Determines whether two access rights are equal.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether two access rights are equal.
+		/// </remarks>
+		/// <returns><c>true</c> if <paramref name="right1"/> and <paramref name="right2"/> are equal; otherwise, <c>false</c>.</returns>
+		/// <param name="right1">The first access right to compare.</param>
+		/// <param name="right2">The second access right to compare.</param>
+		public static bool operator == (AccessRight right1, AccessRight right2)
+		{
+			return right1.Right == right2.Right;
+		}
+
+		/// <summary>
+		/// Determines whether two access rights are not equal.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether two access rights are not equal.
+		/// </remarks>
+		/// <returns><c>true</c> if <paramref name="right1"/> and <paramref name="right2"/> are not equal; otherwise, <c>false</c>.</returns>
+		/// <param name="right1">The first access right to compare.</param>
+		/// <param name="right2">The second access right to compare.</param>
+		public static bool operator != (AccessRight right1, AccessRight right2)
+		{
+			return right1.Right != right2.Right;
+		}
+
+		/// <summary>
+		/// Determines whether the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.AccessRight"/>.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.AccessRight"/>.
+		/// </remarks>
+		/// <param name="obj">The <see cref="System.Object"/> to compare with the current <see cref="MailKit.AccessRight"/>.</param>
+		/// <returns><c>true</c> if the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.AccessRight"/>;
+		/// otherwise, <c>false</c>.</returns>
+		public override bool Equals (object obj)
+		{
+			return obj is AccessRight && ((AccessRight) obj).Right == Right;
+		}
+
+		/// <summary>
+		/// Serves as a hash function for a <see cref="MailKit.AccessRight"/> object.
+		/// </summary>
+		/// <remarks>
+		/// Serves as a hash function for a <see cref="MailKit.AccessRight"/> object.
+		/// </remarks>
+		/// <returns>A hash code for this instance that is suitable for use in hashing algorithms and data structures such as a hash table.</returns>
+		public override int GetHashCode ()
+		{
+			return Right.GetHashCode ();
+		}
+
+		/// <summary>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.AccessRight"/>.
+		/// </summary>
+		/// <remarks>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.AccessRight"/>.
+		/// </remarks>
+		/// <returns>A <see cref="System.String"/> that represents the current <see cref="MailKit.AccessRight"/>.</returns>
+		public override string ToString ()
+		{
+			return Right.ToString ();
+		}
+	}
+}

src/MailKit/AccessRights.cs

@@ -0,0 +1,317 @@
+//
+// AccessRights.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace MailKit {
+	/// <summary>
+	/// A set of access rights.
+	/// </summary>
+	/// <remarks>
+	/// The set of access rights for a particular identity.
+	/// </remarks>
+	public class AccessRights : ICollection<AccessRight>
+	{
+		readonly List<AccessRight> list = new List<AccessRight> ();
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AccessRights"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new set of access rights.
+		/// </remarks>
+		/// <param name="rights">The access rights.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="rights"/> is <c>null</c>.
+		/// </exception>
+		public AccessRights (IEnumerable<AccessRight> rights)
+		{
+			AddRange (rights);
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AccessRights"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new set of access rights.
+		/// </remarks>
+		/// <param name="rights">The access rights.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="rights"/> is <c>null</c>.
+		/// </exception>
+		public AccessRights (string rights)
+		{
+			AddRange (rights);
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AccessRights"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates an empty set of access rights.
+		/// </remarks>
+		public AccessRights ()
+		{
+		}
+
+		/// <summary>
+		/// Get the number of access rights in the collection.
+		/// </summary>
+		/// <remarks>
+		/// Gets the number of access rights in the collection.
+		/// </remarks>
+		/// <value>The count.</value>
+		public int Count {
+			get { return list.Count; }
+		}
+
+		/// <summary>
+		/// Get whether or not this set of access rights is read only.
+		/// </summary>
+		/// <remarks>
+		/// Gets whether or not this set of access rights is read only.
+		/// </remarks>
+		/// <value><c>true</c> if this collection is read only; otherwise, <c>false</c>.</value>
+		public bool IsReadOnly {
+			get { return false; }
+		}
+
+		/// <summary>
+		/// Add the specified access right.
+		/// </summary>
+		/// <remarks>
+		/// Adds the specified access right if it is not already included.
+		/// </remarks>
+		/// <param name="right">The access right.</param>
+		void ICollection<AccessRight>.Add (AccessRight right)
+		{
+			Add (right);
+		}
+
+		/// <summary>
+		/// Add the specified access right.
+		/// </summary>
+		/// <remarks>
+		/// Adds the specified access right if it is not already included.
+		/// </remarks>
+		/// <returns><c>true</c> if the right was added; otherwise, <c>false</c>.</returns>
+		/// <param name="right">The access right.</param>
+		public bool Add (AccessRight right)
+		{
+			if (list.Contains (right))
+				return false;
+
+			list.Add (right);
+
+			return true;
+		}
+
+		/// <summary>
+		/// Add the specified right.
+		/// </summary>
+		/// <remarks>
+		/// Adds the right specified by the given character.
+		/// </remarks>
+		/// <returns><c>true</c> if the right was added; otherwise, <c>false</c>.</returns>
+		/// <param name="right">The right.</param>
+		public bool Add (char right)
+		{
+			return Add (new AccessRight (right));
+		}
+
+		/// <summary>
+		/// Add the rights specified by the characters in the given string.
+		/// </summary>
+		/// <remarks>
+		/// Adds the rights specified by the characters in the given string.
+		/// </remarks>
+		/// <param name="rights">The rights.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="rights"/> is <c>null</c>.
+		/// </exception>
+		public void AddRange (string rights)
+		{
+			if (rights == null)
+				throw new ArgumentNullException (nameof (rights));
+
+			for (int i = 0; i < rights.Length; i++)
+				Add (new AccessRight (rights[i]));
+		}
+
+		/// <summary>
+		/// Add the range of specified rights.
+		/// </summary>
+		/// <remarks>
+		/// Adds the range of specified rights.
+		/// </remarks>
+		/// <param name="rights">The rights.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="rights"/> is <c>null</c>.
+		/// </exception>
+		public void AddRange (IEnumerable<AccessRight> rights)
+		{
+			if (rights == null)
+				throw new ArgumentNullException (nameof (rights));
+
+			foreach (var right in rights)
+				Add (right);
+		}
+
+		/// <summary>
+		/// Clears the access rights.
+		/// </summary>
+		/// <remarks>
+		/// Removes all of the access rights.
+		/// </remarks>
+		public void Clear ()
+		{
+			list.Clear ();
+		}
+
+		/// <summary>
+		/// Checks if the set of access rights contains the specified right.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether or not the set of access rights already contains the specified right
+		/// </remarks>
+		/// <returns><value>true</value> if the specified right exists; otherwise <value>false</value>.</returns>
+		/// <param name="right">The access right.</param>
+		public bool Contains (AccessRight right)
+		{
+			return list.Contains (right);
+		}
+
+		/// <summary>
+		/// Copies all of the access rights to the specified array.
+		/// </summary>
+		/// <remarks>
+		/// Copies all of the access rights into the array,
+		/// starting at the specified array index.
+		/// </remarks>
+		/// <param name="array">The array.</param>
+		/// <param name="arrayIndex">The array index.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="array"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="arrayIndex"/> is out of range.
+		/// </exception>
+		public void CopyTo (AccessRight[] array, int arrayIndex)
+		{
+			if (array == null)
+				throw new ArgumentNullException (nameof (array));
+
+			if (arrayIndex < 0 || arrayIndex + Count > array.Length)
+				throw new ArgumentOutOfRangeException (nameof (arrayIndex));
+
+			list.CopyTo (array, arrayIndex);
+		}
+
+		/// <summary>
+		/// Removes the specified access right.
+		/// </summary>
+		/// <remarks>
+		/// Removes the specified access right.
+		/// </remarks>
+		/// <returns><value>true</value> if the access right was removed; otherwise <value>false</value>.</returns>
+		/// <param name="right">The access right.</param>
+		public bool Remove (AccessRight right)
+		{
+			return list.Remove (right);
+		}
+
+		/// <summary>
+		/// Get the access right at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Gets the access right at the specified index.
+		/// </remarks>
+		/// <value>The access right at the specified index.</value>
+		/// <param name="index">The index.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public AccessRight this [int index] {
+			get {
+				if (index < 0 || index >= list.Count)
+					throw new ArgumentOutOfRangeException (nameof (index));
+
+				return list[index];
+			}
+		}
+
+		#region IEnumerable implementation
+
+		/// <summary>
+		/// Get the access rights enumerator.
+		/// </summary>
+		/// <remarks>
+		/// Gets the access rights enumerator.
+		/// </remarks>
+		/// <returns>The enumerator.</returns>
+		public IEnumerator<AccessRight> GetEnumerator ()
+		{
+			return list.GetEnumerator ();
+		}
+
+		#endregion
+
+		#region IEnumerable implementation
+
+		/// <summary>
+		/// Get the access rights enumerator.
+		/// </summary>
+		/// <remarks>
+		/// Gets the access rights enumerator.
+		/// </remarks>
+		/// <returns>The enumerator.</returns>
+		IEnumerator IEnumerable.GetEnumerator ()
+		{
+			return list.GetEnumerator ();
+		}
+
+		#endregion
+
+		/// <summary>
+		/// Return a <see cref="System.String"/> that represents the current <see cref="MailKit.AccessRights"/>.
+		/// </summary>
+		/// <remarks>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.AccessRights"/>.
+		/// </remarks>
+		/// <returns>A <see cref="System.String"/> that represents the current <see cref="MailKit.AccessRights"/>.</returns>
+		public override string ToString ()
+		{
+			var rights = new char[list.Count];
+
+			for (int i = 0; i < list.Count; i++)
+				rights[i] = list[i].Right;
+
+			return new string (rights);
+		}
+	}
+}

src/MailKit/AlertEventArgs.cs

@@ -0,0 +1,69 @@
+//
+// AlertEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// Alert event arguments.
+	/// </summary>
+	/// <remarks>
+	/// Some <see cref="IMailStore"/> implementations, such as
+	/// <see cref="MailKit.Net.Imap.ImapClient"/>, will emit Alert
+	/// events when they receive alert messages from the server.
+	/// </remarks>
+	public class AlertEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AlertEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="AlertEventArgs"/>.
+		/// </remarks>
+		/// <param name="message">The alert message.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="message"/> is <c>null</c>.
+		/// </exception>
+		public AlertEventArgs (string message)
+		{
+			if (message == null)
+				throw new ArgumentNullException (nameof (message));
+
+			Message = message;
+		}
+
+		/// <summary>
+		/// Gets the alert message.
+		/// </summary>
+		/// <remarks>
+		/// The alert message will be the exact message received from the server.
+		/// </remarks>
+		/// <value>The alert message.</value>
+		public string Message {
+			get; private set;
+		}
+	}
+}

src/MailKit/Annotation.cs

@@ -0,0 +1,81 @@
+//
+// Annotation.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Collections.Generic;
+
+namespace MailKit {
+	/// <summary>
+	/// An annotation.
+	/// </summary>
+	/// <remarks>
+	/// <para>An annotation.</para>
+	/// <para>For more information about annotations, see
+	/// <a href="https://tools.ietf.org/html/rfc5257">rfc5257</a>.</para>
+	/// </remarks>
+	public class Annotation
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Annotation"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="Annotation"/>.
+		/// </remarks>
+		/// <param name="entry">The annotation entry.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="entry"/> is <c>null</c>.
+		/// </exception>
+		public Annotation (AnnotationEntry entry)
+		{
+			if (entry == null)
+				throw new ArgumentNullException (nameof (entry));
+
+			Properties = new Dictionary<AnnotationAttribute, string> ();
+			Entry = entry;
+		}
+
+		/// <summary>
+		/// Get the annotation tag.
+		/// </summary>
+		/// <remarks>
+		/// Gets the annotation tag.
+		/// </remarks>
+		/// <value>The annotation tag.</value>
+		public AnnotationEntry Entry {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the annotation properties.
+		/// </summary>
+		/// <remarks>
+		/// Gets the annotation properties.
+		/// </remarks>
+		public Dictionary<AnnotationAttribute, string> Properties {
+			get; private set;
+		}
+	}
+}

src/MailKit/AnnotationAccess.cs

@@ -0,0 +1,53 @@
+//
+// AnnotationAccess.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit {
+	/// <summary>
+	/// An annotation access level.
+	/// </summary>
+	/// <remarks>
+	/// <para>An annotation access level.</para>
+	/// <para>For more information about annotations, see
+	/// <a href="https://tools.ietf.org/html/rfc5257">rfc5257</a>.</para>
+	/// </remarks>
+	public enum AnnotationAccess
+	{
+		/// <summary>
+		/// Annotations are not supported.
+		/// </summary>
+		None,
+
+		/// <summary>
+		/// Annotations are read-only.
+		/// </summary>
+		ReadOnly,
+
+		/// <summary>
+		/// Annotations are read-write.
+		/// </summary>
+		ReadWrite
+	}
+}

src/MailKit/AnnotationAttribute.cs

@@ -0,0 +1,251 @@
+//
+// AnnotationAttribute.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// An annotation attribute.
+	/// </summary>
+	/// <remarks>
+	/// <para>An annotation attribute.</para>
+	/// <para>For more information about annotations, see
+	/// <a href="https://tools.ietf.org/html/rfc5257">rfc5257</a>.</para>
+	/// </remarks>
+	public class AnnotationAttribute : IEquatable<AnnotationAttribute>
+	{
+		static readonly char[] Wildcards = { '*', '%' };
+
+		/// <summary>
+		/// The annotation value.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set both the private and shared values of an annotation.
+		/// </remarks>
+		public static readonly AnnotationAttribute Value = new AnnotationAttribute ("value", AnnotationScope.Both);
+
+		/// <summary>
+		/// The shared annotation value.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set the shared value of an annotation.
+		/// </remarks>
+		public static readonly AnnotationAttribute SharedValue = new AnnotationAttribute ("value", AnnotationScope.Shared);
+
+		/// <summary>
+		/// The private annotation value.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set the private value of an annotation.
+		/// </remarks>
+		public static readonly AnnotationAttribute PrivateValue = new AnnotationAttribute ("value", AnnotationScope.Private);
+
+		/// <summary>
+		/// The size of an annotation value.
+		/// </summary>
+		/// <remarks>
+		/// Used to get the size of the both the private and shared annotation values.
+		/// </remarks>
+		public static readonly AnnotationAttribute Size = new AnnotationAttribute ("size", AnnotationScope.Both);
+
+		/// <summary>
+		/// The size of a shared annotation value.
+		/// </summary>
+		/// <remarks>
+		/// Used to get the size of a shared annotation value.
+		/// </remarks>
+		public static readonly AnnotationAttribute SharedSize = new AnnotationAttribute ("size", AnnotationScope.Shared);
+
+		/// <summary>
+		/// The size of a private annotation value.
+		/// </summary>
+		/// <remarks>
+		/// Used to get the size of a private annotation value.
+		/// </remarks>
+		public static readonly AnnotationAttribute PrivateSize = new AnnotationAttribute ("size", AnnotationScope.Private);
+
+		AnnotationAttribute (string name, AnnotationScope scope)
+		{
+			switch (scope) {
+			case AnnotationScope.Shared: Specifier = string.Format ("{0}.shared", name); break;
+			case AnnotationScope.Private: Specifier = string.Format ("{0}.priv", name); break;
+			default: Specifier = name; break;
+			}
+			Scope = scope;
+			Name = name;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AnnotationAttribute"/> class.
+		/// </summary>
+		/// <param name="specifier">The annotation attribute specifier.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="specifier"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="specifier"/> contains illegal characters.
+		/// </exception>
+		public AnnotationAttribute (string specifier)
+		{
+			if (specifier == null)
+				throw new ArgumentNullException (nameof (specifier));
+
+			if (specifier.Length == 0)
+				throw new ArgumentException ("Annotation attribute specifiers cannot be empty.", nameof (specifier));
+
+			// TODO: improve validation
+			if (specifier.IndexOfAny (Wildcards) != -1)
+				throw new ArgumentException ("Annotation attribute specifiers cannot contain '*' or '%'.", nameof (specifier));
+
+			Specifier = specifier;
+
+			if (specifier.EndsWith (".shared", StringComparison.Ordinal)) {
+				Name = specifier.Substring (0, specifier.Length - ".shared".Length);
+				Scope = AnnotationScope.Shared;
+			} else if (specifier.EndsWith (".priv", StringComparison.Ordinal)) {
+				Name = specifier.Substring (0, specifier.Length - ".priv".Length);
+				Scope = AnnotationScope.Private;
+			} else {
+				Scope = AnnotationScope.Both;
+				Name = specifier;
+			}
+		}
+
+		/// <summary>
+		/// Get the name of the annotation attribute.
+		/// </summary>
+		/// <remarks>
+		/// Gets the name of the annotation attribute.
+		/// </remarks>
+		public string Name {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the scope of the annotation attribute.
+		/// </summary>
+		/// <remarks>
+		/// Gets the scope of the annotation attribute.
+		/// </remarks>
+		public AnnotationScope Scope {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the annotation attribute specifier.
+		/// </summary>
+		/// <remarks>
+		/// Gets the annotation attribute specifier.
+		/// </remarks>
+		public string Specifier {
+			get; private set;
+		}
+
+		#region IEquatable implementation
+
+		/// <summary>
+		/// Determines whether the specified <see cref="MailKit.AnnotationAttribute"/> is equal to the current <see cref="MailKit.AnnotationAttribute"/>.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether the specified <see cref="MailKit.AnnotationAttribute"/> is equal to the current <see cref="MailKit.AnnotationAttribute"/>.
+		/// </remarks>
+		/// <param name="other">The <see cref="MailKit.AnnotationAttribute"/> to compare with the current <see cref="MailKit.AnnotationAttribute"/>.</param>
+		/// <returns><c>true</c> if the specified <see cref="MailKit.AnnotationAttribute"/> is equal to the current
+		/// <see cref="MailKit.AnnotationAttribute"/>; otherwise, <c>false</c>.</returns>
+		public bool Equals (AnnotationAttribute other)
+		{
+			return other?.Specifier == Specifier;
+		}
+
+		#endregion
+
+		/// <summary>
+		/// Determines whether two annotation attributes are equal.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether two annotation attributes are equal.
+		/// </remarks>
+		/// <returns><c>true</c> if <paramref name="attr1"/> and <paramref name="attr2"/> are equal; otherwise, <c>false</c>.</returns>
+		/// <param name="attr1">The first annotation attribute to compare.</param>
+		/// <param name="attr2">The second annotation attribute to compare.</param>
+		public static bool operator == (AnnotationAttribute attr1, AnnotationAttribute attr2)
+		{
+			return attr1?.Specifier == attr2?.Specifier;
+		}
+
+		/// <summary>
+		/// Determines whether two annotation attributes are not equal.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether two annotation attributes are not equal.
+		/// </remarks>
+		/// <returns><c>true</c> if <paramref name="attr1"/> and <paramref name="attr2"/> are not equal; otherwise, <c>false</c>.</returns>
+		/// <param name="attr1">The first annotation attribute to compare.</param>
+		/// <param name="attr2">The second annotation attribute to compare.</param>
+		public static bool operator != (AnnotationAttribute attr1, AnnotationAttribute attr2)
+		{
+			return attr1?.Specifier != attr2?.Specifier;
+		}
+
+		/// <summary>
+		/// Determine whether the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.AnnotationAttribute"/>.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.AnnotationAttribute"/>.
+		/// </remarks>
+		/// <param name="obj">The <see cref="System.Object"/> to compare with the current <see cref="MailKit.AnnotationAttribute"/>.</param>
+		/// <returns><c>true</c> if the specified <see cref="System.Object"/> is equal to the current
+		/// <see cref="MailKit.AnnotationAttribute"/>; otherwise, <c>false</c>.</returns>
+		public override bool Equals (object obj)
+		{
+			return obj is AnnotationAttribute && ((AnnotationAttribute) obj).Specifier == Specifier;
+		}
+
+		/// <summary>
+		/// Serves as a hash function for a <see cref="MailKit.AnnotationAttribute"/> object.
+		/// </summary>
+		/// <remarks>
+		/// Serves as a hash function for a <see cref="MailKit.AnnotationAttribute"/> object.
+		/// </remarks>
+		/// <returns>A hash code for this instance that is suitable for use in hashing algorithms and data structures such as a hash table.</returns>
+		public override int GetHashCode ()
+		{
+			return Specifier.GetHashCode ();
+		}
+
+		/// <summary>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.AnnotationAttribute"/>.
+		/// </summary>
+		/// <remarks>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.AnnotationAttribute"/>.
+		/// </remarks>
+		/// <returns>A <see cref="System.String"/> that represents the current <see cref="MailKit.AnnotationAttribute"/>.</returns>
+		public override string ToString ()
+		{
+			return Specifier;
+		}
+	}
+}

src/MailKit/AnnotationEntry.cs

@@ -0,0 +1,521 @@
+//
+// Annotationentry.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// An annotation entry.
+	/// </summary>
+	/// <remarks>
+	/// <para>An annotation entry.</para>
+	/// <para>For more information about annotations, see
+	/// <a href="https://tools.ietf.org/html/rfc5257">rfc5257</a>.</para>
+	/// </remarks>
+	public class AnnotationEntry : IEquatable<AnnotationEntry>
+	{
+		/// <summary>
+		/// An annotation entry for a comment on a message.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set a comment on a message.
+		/// </remarks>
+		public static readonly AnnotationEntry Comment = new AnnotationEntry ("/comment", AnnotationScope.Both);
+
+		/// <summary>
+		/// An annotation entry for a private comment on a message.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set a private comment on a message.
+		/// </remarks>
+		public static readonly AnnotationEntry PrivateComment = new AnnotationEntry ("/comment", AnnotationScope.Private);
+
+		/// <summary>
+		/// An annotation entry for a shared comment on a message.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set a shared comment on a message.
+		/// </remarks>
+		public static readonly AnnotationEntry SharedComment = new AnnotationEntry ("/comment", AnnotationScope.Shared);
+
+		/// <summary>
+		/// An annotation entry for flags on a message.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set flags on a message.
+		/// </remarks>
+		public static readonly AnnotationEntry Flags = new AnnotationEntry ("/flags", AnnotationScope.Both);
+
+		/// <summary>
+		/// An annotation entry for private flags on a message.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set private flags on a message.
+		/// </remarks>
+		public static readonly AnnotationEntry PrivateFlags = new AnnotationEntry ("/flags", AnnotationScope.Private);
+
+		/// <summary>
+		/// Aa annotation entry for shared flags on a message.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set shared flags on a message.
+		/// </remarks>
+		public static readonly AnnotationEntry SharedFlags = new AnnotationEntry ("/flags", AnnotationScope.Shared);
+
+		/// <summary>
+		/// An annotation entry for an alternate subject on a message.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set an alternate subject on a message.
+		/// </remarks>
+		public static readonly AnnotationEntry AltSubject = new AnnotationEntry ("/altsubject", AnnotationScope.Both);
+
+		/// <summary>
+		/// An annotation entry for a private alternate subject on a message.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set a private alternate subject on a message.
+		/// </remarks>
+		public static readonly AnnotationEntry PrivateAltSubject = new AnnotationEntry ("/altsubject", AnnotationScope.Private);
+
+		/// <summary>
+		/// An annotation entry for a shared alternate subject on a message.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set a shared alternate subject on a message.
+		/// </remarks>
+		public static readonly AnnotationEntry SharedAltSubject = new AnnotationEntry ("/altsubject", AnnotationScope.Shared);
+
+		static void ValidatePath (string path)
+		{
+			if (path == null)
+				throw new ArgumentNullException (nameof (path));
+
+			if (path.Length == 0)
+				throw new ArgumentException ("Annotation entry paths cannot be empty.", nameof (path));
+
+			if (path[0] != '/' && path[0] != '*' && path[0] != '%')
+				throw new ArgumentException ("Annotation entry paths must begin with '/'.", nameof (path));
+
+			if (path.Length > 1 && path[1] >= '0' && path[1] <= '9')
+				throw new ArgumentException ("Annotation entry paths must not include a part-specifier.", nameof (path));
+
+			if (path == "*" || path == "%")
+				return;
+
+			char pc = path[0];
+
+			for (int i = 1; i < path.Length; i++) {
+				char c = path[i];
+
+				if (c > 127)
+					throw new ArgumentException ($"Invalid character in annotation entry path: '{c}'.", nameof (path));
+
+				if (c >= '0' && c <= '9' && pc == '/')
+					throw new ArgumentException ("Invalid annotation entry path.", nameof (path));
+
+				if ((pc == '/' || pc == '.') && (c == '/' || c == '.'))
+					throw new ArgumentException ("Invalid annotation entry path.", nameof (path));
+
+				pc = c;
+			}
+
+			int endIndex = path.Length - 1;
+
+			if (path[endIndex] == '/')
+				throw new ArgumentException ("Annotation entry paths must not end with '/'.", nameof (path));
+
+			if (path[endIndex] == '.')
+				throw new ArgumentException ("Annotation entry paths must not end with '.'.", nameof (path));
+		}
+
+		static void ValidatePartSpecifier (string partSpecifier)
+		{
+			if (partSpecifier == null)
+				throw new ArgumentNullException (nameof (partSpecifier));
+
+			char pc = '\0';
+
+			for (int i = 0; i < partSpecifier.Length; i++) {
+				char c = partSpecifier[i];
+
+				if (!((c >= '0' && c <= '9') || c == '.') || (c == '.' && (pc == '.' || pc == '\0')))
+					throw new ArgumentException ("Invalid part-specifier.", nameof (partSpecifier));
+
+				pc = c;
+			}
+
+			if (pc == '.')
+				throw new ArgumentException ("Invalid part-specifier.", nameof (partSpecifier));
+		}
+
+		AnnotationEntry ()
+		{
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AnnotationEntry"/> struct.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="AnnotationEntry"/>.
+		/// </remarks>
+		/// <param name="path">The annotation entry path.</param>
+		/// <param name="scope">The scope of the annotation.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="path"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="path"/> is invalid.
+		/// </exception>
+		public AnnotationEntry (string path, AnnotationScope scope = AnnotationScope.Both)
+		{
+			ValidatePath (path);
+
+			switch (scope) {
+			case AnnotationScope.Private: Entry = path + ".priv"; break;
+			case AnnotationScope.Shared: Entry = path + ".shared"; break;
+			default: Entry = path; break;
+			}
+			PartSpecifier = null;
+			Path = path;
+			Scope = scope;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AnnotationEntry"/> struct.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="AnnotationEntry"/> for an individual body part of a message.
+		/// </remarks>
+		/// <param name="partSpecifier">The part-specifier of the body part of the message.</param>
+		/// <param name="path">The annotation entry path.</param>
+		/// <param name="scope">The scope of the annotation.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="partSpecifier"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="path"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <para><paramref name="partSpecifier"/> is invalid.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="path"/> is invalid.</para>
+		/// </exception>
+		public AnnotationEntry (string partSpecifier, string path, AnnotationScope scope = AnnotationScope.Both)
+		{
+			ValidatePartSpecifier (partSpecifier);
+			ValidatePath (path);
+
+			switch (scope) {
+			case AnnotationScope.Private: Entry = string.Format ("/{0}{1}.priv", partSpecifier, path); break;
+			case AnnotationScope.Shared: Entry = string.Format ("/{0}{1}.shared", partSpecifier, path); break;
+			default: Entry = string.Format ("/{0}{1}", partSpecifier, path); break;
+			}
+			PartSpecifier = partSpecifier;
+			Path = path;
+			Scope = scope;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AnnotationEntry"/> struct.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="AnnotationEntry"/> for an individual body part of a message.
+		/// </remarks>
+		/// <param name="part">The body part of the message.</param>
+		/// <param name="path">The annotation entry path.</param>
+		/// <param name="scope">The scope of the annotation.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="part"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="path"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="path"/> is invalid.
+		/// </exception>
+		public AnnotationEntry (BodyPart part, string path, AnnotationScope scope = AnnotationScope.Both)
+		{
+			if (part == null)
+				throw new ArgumentNullException (nameof (part));
+
+			ValidatePath (path);
+
+			switch (scope) {
+			case AnnotationScope.Private: Entry = string.Format ("/{0}{1}.priv", part.PartSpecifier, path); break;
+			case AnnotationScope.Shared: Entry = string.Format ("/{0}{1}.shared", part.PartSpecifier, path); break;
+			default: Entry = string.Format ("/{0}{1}", part.PartSpecifier, path); break;
+			}
+			PartSpecifier = part.PartSpecifier;
+			Path = path;
+			Scope = scope;
+		}
+
+		/// <summary>
+		/// Get the annotation entry specifier.
+		/// </summary>
+		/// <remarks>
+		/// Gets the annotation entry specifier.
+		/// </remarks>
+		/// <value>The annotation entry specifier.</value>
+		public string Entry {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the part-specifier component of the annotation entry.
+		/// </summary>
+		/// <remarks>
+		/// Gets the part-specifier component of the annotation entry.
+		/// </remarks>
+		public string PartSpecifier {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the path component of the annotation entry.
+		/// </summary>
+		/// <remarks>
+		/// Gets the path component of the annotation entry.
+		/// </remarks>
+		public string Path {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the scope of the annotation.
+		/// </summary>
+		/// <remarks>
+		/// Gets the scope of the annotation.
+		/// </remarks>
+		public AnnotationScope Scope {
+			get; private set;
+		}
+
+		#region IEquatable implementation
+
+		/// <summary>
+		/// Determines whether the specified <see cref="MailKit.AnnotationEntry"/> is equal to the current <see cref="MailKit.AnnotationEntry"/>.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether the specified <see cref="MailKit.AnnotationEntry"/> is equal to the current <see cref="MailKit.AnnotationEntry"/>.
+		/// </remarks>
+		/// <param name="other">The <see cref="MailKit.AnnotationEntry"/> to compare with the current <see cref="MailKit.AnnotationEntry"/>.</param>
+		/// <returns><c>true</c> if the specified <see cref="MailKit.AnnotationEntry"/> is equal to the current
+		/// <see cref="MailKit.AnnotationEntry"/>; otherwise, <c>false</c>.</returns>
+		public bool Equals (AnnotationEntry other)
+		{
+			return other?.Entry == Entry;
+		}
+
+		#endregion
+
+		/// <summary>
+		/// Determines whether two annotation entries are equal.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether two annotation entries are equal.
+		/// </remarks>
+		/// <returns><c>true</c> if <paramref name="entry1"/> and <paramref name="entry2"/> are equal; otherwise, <c>false</c>.</returns>
+		/// <param name="entry1">The first annotation entry to compare.</param>
+		/// <param name="entry2">The second annotation entry to compare.</param>
+		public static bool operator == (AnnotationEntry entry1, AnnotationEntry entry2)
+		{
+			return entry1?.Entry == entry2?.Entry;
+		}
+
+		/// <summary>
+		/// Determines whether two annotation entries are not equal.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether two annotation entries are not equal.
+		/// </remarks>
+		/// <returns><c>true</c> if <paramref name="entry1"/> and <paramref name="entry2"/> are not equal; otherwise, <c>false</c>.</returns>
+		/// <param name="entry1">The first annotation entry to compare.</param>
+		/// <param name="entry2">The second annotation entry to compare.</param>
+		public static bool operator != (AnnotationEntry entry1, AnnotationEntry entry2)
+		{
+			return entry1?.Entry != entry2?.Entry;
+		}
+
+		/// <summary>
+		/// Determine whether the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.AnnotationEntry"/>.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.AnnotationEntry"/>.
+		/// </remarks>
+		/// <param name="obj">The <see cref="System.Object"/> to compare with the current <see cref="MailKit.AnnotationEntry"/>.</param>
+		/// <returns><c>true</c> if the specified <see cref="System.Object"/> is equal to the current
+		/// <see cref="MailKit.AnnotationEntry"/>; otherwise, <c>false</c>.</returns>
+		public override bool Equals (object obj)
+		{
+			return obj is AnnotationEntry && ((AnnotationEntry) obj).Entry == Entry;
+		}
+
+		/// <summary>
+		/// Serves as a hash function for a <see cref="MailKit.AnnotationEntry"/> object.
+		/// </summary>
+		/// <remarks>
+		/// Serves as a hash function for a <see cref="MailKit.AnnotationEntry"/> object.
+		/// </remarks>
+		/// <returns>A hash code for this instance that is suitable for use in hashing algorithms and data structures such as a hash table.</returns>
+		public override int GetHashCode ()
+		{
+			return Entry.GetHashCode ();
+		}
+
+		/// <summary>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.AnnotationEntry"/>.
+		/// </summary>
+		/// <remarks>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.AnnotationEntry"/>.
+		/// </remarks>
+		/// <returns>A <see cref="System.String"/> that represents the current <see cref="MailKit.AnnotationEntry"/>.</returns>
+		public override string ToString ()
+		{
+			return Entry;
+		}
+
+		/// <summary>
+		/// Parse an annotation entry.
+		/// </summary>
+		/// <remarks>
+		/// Parses an annotation entry.
+		/// </remarks>
+		/// <param name="entry">The annotation entry.</param>
+		/// <returns>The parsed annotation entry.</returns>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="entry"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.FormatException">
+		/// <paramref name="entry"/> does not conform to the annotation entry syntax.
+		/// </exception>
+		public static AnnotationEntry Parse (string entry)
+		{
+			if (entry == null)
+				throw new ArgumentNullException (nameof (entry));
+
+			if (entry.Length == 0)
+				throw new FormatException ("An annotation entry cannot be empty.");
+
+			if (entry[0] != '/' && entry[0] != '*' && entry[0] != '%')
+				throw new FormatException ("An annotation entry must begin with a '/' character.");
+
+			var scope = AnnotationScope.Both;
+			int startIndex = 0, endIndex;
+			string partSpecifier = null;
+			var component = 0;
+			var pc = entry[0];
+			string path;
+
+			for (int i = 1; i < entry.Length; i++) {
+				char c = entry[i];
+
+				if (c >= '0' && c <= '9' && pc == '/') {
+					if (component > 0)
+						throw new FormatException ("Invalid annotation entry.");
+
+					startIndex = i;
+					endIndex = i + 1;
+					pc = c;
+
+					while (endIndex < entry.Length) {
+						c = entry[endIndex];
+
+						if (c == '/') {
+							if (pc == '.')
+								throw new FormatException ("Invalid part-specifier in annotation entry.");
+
+							break;
+						}
+
+						if (!(c >= '0' && c <= '9') && c != '.')
+							throw new FormatException ($"Invalid character in part-specifier: '{c}'.");
+
+						if (c == '.' && pc == '.')
+							throw new FormatException ("Invalid part-specifier in annotation entry.");
+
+						endIndex++;
+						pc = c;
+					}
+
+					if (endIndex >= entry.Length)
+						throw new FormatException ("Incomplete part-specifier in annotation entry.");
+
+					partSpecifier = entry.Substring (startIndex, endIndex - startIndex);
+					i = startIndex = endIndex;
+					component++;
+				} else if (c == '/' || c == '.') {
+					if (pc == '/' || pc == '.')
+						throw new FormatException ("Invalid annotation entry path.");
+
+					if (c == '/')
+						component++;
+				} else if (c > 127) {
+					throw new FormatException ($"Invalid character in annotation entry path: '{c}'.");
+				}
+
+				pc = c;
+			}
+
+			if (pc == '/' || pc == '.')
+				throw new FormatException ("Invalid annotation entry path.");
+
+			if (entry.EndsWith (".shared", StringComparison.Ordinal)) {
+				endIndex = entry.Length - ".shared".Length;
+				scope = AnnotationScope.Shared;
+			} else if (entry.EndsWith (".priv", StringComparison.Ordinal)) {
+				endIndex = entry.Length - ".priv".Length;
+				scope = AnnotationScope.Private;
+			} else {
+				endIndex = entry.Length;
+			}
+
+			path = entry.Substring (startIndex, endIndex - startIndex);
+
+			return new AnnotationEntry {
+				PartSpecifier = partSpecifier,
+				Entry = entry,
+				Path = path,
+				Scope = scope
+			};
+		}
+
+		internal static AnnotationEntry Create (string entry)
+		{
+			switch (entry) {
+			case "/comment": return Comment;
+			case "/comment.priv": return PrivateComment;
+			case "/comment.shared": return SharedComment;
+			case "/flags": return Flags;
+			case "/flags.priv": return PrivateFlags;
+			case "/flags.shared": return SharedFlags;
+			case "/altsubject": return AltSubject;
+			case "/altsubject.priv": return PrivateAltSubject;
+			case "/altsubject.shared": return SharedAltSubject;
+			default: return Parse (entry);
+			}
+		}
+	}
+}

src/MailKit/AnnotationScope.cs

@@ -0,0 +1,61 @@
+//
+// AnnotationScope.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// The scope of an annotation.
+	/// </summary>
+	/// <remarks>
+	/// <para>Represents the scope of an annotation.</para>
+	/// <para>For more information about annotations, see
+	/// <a href="https://tools.ietf.org/html/rfc5257">rfc5257</a>.</para>
+	/// </remarks>
+	[Flags]
+	public enum AnnotationScope
+	{
+		/// <summary>
+		/// No scopes.
+		/// </summary>
+		None,
+
+		/// <summary>
+		/// The private annotation scope.
+		/// </summary>
+		Private,
+
+		/// <summary>
+		/// The shared annotation scope.
+		/// </summary>
+		Shared,
+
+		/// <summary>
+		/// Both private and shared scopes.
+		/// </summary>
+		Both = Private | Shared
+	}
+}

src/MailKit/AnnotationsChangedEventArgs.cs

@@ -0,0 +1,93 @@
+//
+// AnnotationsChangedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Linq;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args used when an annotation changes.
+	/// </summary>
+	/// <remarks>
+	/// Event args used when an annotation changes.
+	/// </remarks>
+	public class AnnotationsChangedEventArgs : MessageEventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AnnotationsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="AnnotationsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		internal AnnotationsChangedEventArgs (int index) : base (index)
+		{
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.AnnotationsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="AnnotationsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="annotations">The annotations that changed.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="annotations"/> is <c>null</c>.
+		/// </exception>
+		public AnnotationsChangedEventArgs (int index, IEnumerable<Annotation> annotations) : base (index)
+		{
+			if (annotations == null)
+				throw new ArgumentNullException (nameof (annotations));
+
+			Annotations = new ReadOnlyCollection<Annotation> (annotations.ToArray ());
+		}
+
+		/// <summary>
+		/// Get the annotations that changed.
+		/// </summary>
+		/// <remarks>
+		/// Gets the annotations that changed.
+		/// </remarks>
+		/// <value>The annotation.</value>
+		public IList<Annotation> Annotations {
+			get; internal set;
+		}
+
+		/// <summary>
+		/// Gets the updated mod-sequence value of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the updated mod-sequence value of the message, if available.
+		/// </remarks>
+		/// <value>The mod-sequence value.</value>
+		public ulong? ModSeq {
+			get; internal set;
+		}
+	}
+}

src/MailKit/AuthenticatedEventArgs.cs

@@ -0,0 +1,68 @@
+//
+// AuthenticatedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// Authenticated event arguments.
+	/// </summary>
+	/// <remarks>
+	/// Some servers, such as GMail IMAP, will send some free-form text in
+	/// the response to a successful login.
+	/// </remarks>
+	public class AuthenticatedEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.AuthenticatedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="AuthenticatedEventArgs"/>.
+		/// </remarks>
+		/// <param name="message">The free-form text.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="message"/> is <c>null</c>.
+		/// </exception>
+		public AuthenticatedEventArgs (string message)
+		{
+			if (message == null)
+				throw new ArgumentNullException (nameof (message));
+
+			Message = message;
+		}
+
+		/// <summary>
+		/// Get the free-form text sent by the server.
+		/// </summary>
+		/// <remarks>
+		/// Gets the free-form text sent by the server.
+		/// </remarks>
+		/// <value>The free-form text sent by the server.</value>
+		public string Message {
+			get; private set;
+		}
+	}
+}

src/MailKit/BodyPart.cs

@@ -0,0 +1,716 @@
+//
+// BodyPart.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+using System.Collections.Generic;
+
+using MimeKit;
+using MimeKit.Utils;
+
+namespace MailKit {
+	/// <summary>
+	/// An abstract body part of a message.
+	/// </summary>
+	/// <remarks>
+	/// Each body part will actually be a <see cref="BodyPartBasic"/>,
+	/// <see cref="BodyPartText"/>, <see cref="BodyPartMessage"/>, or
+	/// <see cref="BodyPartMultipart"/>.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadBodyParts"/>
+	/// </example>
+	public abstract class BodyPart
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.BodyPart"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.BodyPart"/>.
+		/// </remarks>
+		protected BodyPart ()
+		{
+		}
+
+		/// <summary>
+		/// Gets the Content-Type of the body part.
+		/// </summary>
+		/// <remarks>
+		/// Gets the Content-Type of the body part.
+		/// </remarks>
+		/// <value>The content type.</value>
+		public ContentType ContentType {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the part specifier.
+		/// </summary>
+		/// <remarks>
+		/// Gets the part specifier.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadBodyParts"/>
+		/// </example>
+		/// <value>The part specifier.</value>
+		public string PartSpecifier {
+			get; set;
+		}
+
+		/// <summary>
+		/// Dispatches to the specific visit method for this MIME body part.
+		/// </summary>
+		/// <remarks>
+		/// This default implementation for <see cref="MailKit.BodyPart"/> nodes
+		/// calls <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>. Override this
+		/// method to call into a more specific method on a derived visitor class
+		/// of the <see cref="MailKit.BodyPartVisitor"/> class. However, it should still
+		/// support unknown visitors by calling
+		/// <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>.
+		/// </remarks>
+		/// <param name="visitor">The visitor.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="visitor"/> is <c>null</c>.
+		/// </exception>
+		public abstract void Accept (BodyPartVisitor visitor);
+
+		internal static void Encode (StringBuilder builder, uint value)
+		{
+			builder.Append (value.ToString ());
+		}
+
+		internal static void Encode (StringBuilder builder, string value)
+		{
+			if (value != null)
+				builder.Append (MimeUtils.Quote (value));
+			else
+				builder.Append ("NIL");
+		}
+
+		internal static void Encode (StringBuilder builder, Uri location)
+		{
+			if (location != null)
+				builder.Append (MimeUtils.Quote (location.ToString ()));
+			else
+				builder.Append ("NIL");
+		}
+
+		internal static void Encode (StringBuilder builder, string[] values)
+		{
+			if (values == null || values.Length == 0) {
+				builder.Append ("NIL");
+				return;
+			}
+
+			builder.Append ('(');
+
+			for (int i = 0; i < values.Length; i++) {
+				if (i > 0)
+					builder.Append (' ');
+
+				Encode (builder, values[i]);
+			}
+
+			builder.Append (')');
+		}
+
+		internal static void Encode (StringBuilder builder, IList<Parameter> parameters)
+		{
+			if (parameters == null || parameters.Count == 0) {
+				builder.Append ("NIL");
+				return;
+			}
+
+			builder.Append ('(');
+
+			for (int i = 0; i < parameters.Count; i++) {
+				if (i > 0)
+					builder.Append (' ');
+
+				Encode (builder, parameters[i].Name);
+				builder.Append (' ');
+				Encode (builder, parameters[i].Value);
+			}
+
+			builder.Append (')');
+		}
+
+		internal static void Encode (StringBuilder builder, ContentDisposition disposition)
+		{
+			if (disposition == null) {
+				builder.Append ("NIL");
+				return;
+			}
+
+			builder.Append ('(');
+			Encode (builder, disposition.Disposition);
+			builder.Append (' ');
+			Encode (builder, disposition.Parameters);
+			builder.Append (')');
+		}
+
+		internal static void Encode (StringBuilder builder, ContentType contentType)
+		{
+			Encode (builder, contentType.MediaType);
+			builder.Append (' ');
+			Encode (builder, contentType.MediaSubtype);
+			builder.Append (' ');
+			Encode (builder, contentType.Parameters);
+		}
+
+		internal static void Encode (StringBuilder builder, BodyPartCollection parts)
+		{
+			if (parts == null || parts.Count == 0) {
+				builder.Append ("NIL");
+				return;
+			}
+
+			for (int i = 0; i < parts.Count; i++) {
+				if (i > 0)
+					builder.Append (' ');
+
+				Encode (builder, parts[i]);
+			}
+		}
+
+		internal static void Encode (StringBuilder builder, Envelope envelope)
+		{
+			if (envelope == null) {
+				builder.Append ("NIL");
+				return;
+			}
+
+			envelope.Encode (builder);
+		}
+
+		internal static void Encode (StringBuilder builder, BodyPart body)
+		{
+			if (body == null) {
+				builder.Append ("NIL");
+				return;
+			}
+
+			builder.Append ('(');
+			body.Encode (builder);
+			builder.Append (')');
+		}
+
+		/// <summary>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </summary>
+		/// <remarks>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </remarks>
+		/// <param name="builder">The string builder.</param>
+		protected abstract void Encode (StringBuilder builder);
+
+		/// <summary>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.BodyPart"/>.
+		/// </summary>
+		/// <remarks>
+		/// <para>Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.BodyPart"/>.</para>
+		/// <note type="note">The syntax of the string returned, while similar to IMAP's BODYSTRUCTURE syntax,
+		/// is not completely compatible.</note>
+		/// </remarks>
+		/// <returns>A <see cref="System.String"/> that represents the current <see cref="MailKit.BodyPart"/>.</returns>
+		public override string ToString ()
+		{
+			var builder = new StringBuilder ();
+
+			builder.Append ('(');
+			Encode (builder);
+			builder.Append (')');
+
+			return builder.ToString ();
+		}
+
+		static bool TryParse (string text, ref int index, out uint value)
+		{
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			int startIndex = index;
+
+			value = 0;
+
+			while (index < text.Length && char.IsDigit (text[index]))
+				value = (value * 10) + (uint) (text[index++] - '0');
+
+			return index > startIndex;
+		}
+
+		static bool TryParse (string text, ref int index, out string nstring)
+		{
+			nstring = null;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length)
+				return false;
+
+			if (text[index] != '"') {
+				if (index + 3 <= text.Length && text.Substring (index, 3) == "NIL") {
+					index += 3;
+					return true;
+				}
+
+				return false;
+			}
+
+			var token = new StringBuilder ();
+			bool escaped = false;
+
+			index++;
+
+			while (index < text.Length) {
+				if (text[index] == '"' && !escaped)
+					break;
+
+				if (escaped || text[index] != '\\') {
+					token.Append (text[index]);
+					escaped = false;
+				} else {
+					escaped = true;
+				}
+
+				index++;
+			}
+
+			if (index >= text.Length)
+				return false;
+
+			nstring = token.ToString ();
+
+			index++;
+
+			return true;
+		}
+
+		static bool TryParse (string text, ref int index, out string[] values)
+		{
+			values = null;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length)
+				return false;
+
+			if (text[index] != '(') {
+				if (index + 3 <= text.Length && text.Substring (index, 3) == "NIL") {
+					index += 3;
+					return true;
+				}
+
+				return false;
+			}
+
+			index++;
+
+			if (index >= text.Length)
+				return false;
+
+			var list = new List<string> ();
+			string value;
+
+			do {
+				if (text[index] == ')')
+					break;
+
+				if (!TryParse (text, ref index, out value))
+					return false;
+
+				list.Add (value);
+			} while (index < text.Length);
+
+			if (index >= text.Length || text[index] != ')')
+				return false;
+
+			values = list.ToArray ();
+			index++;
+
+			return true;
+		}
+
+		static bool TryParse (string text, ref int index, out Uri uri)
+		{
+			string nstring;
+
+			uri = null;
+
+			if (!TryParse (text, ref index, out nstring))
+				return false;
+
+			if (!string.IsNullOrEmpty (nstring)) {
+				if (Uri.IsWellFormedUriString (nstring, UriKind.Absolute))
+					uri = new Uri (nstring, UriKind.Absolute);
+				else if (Uri.IsWellFormedUriString (nstring, UriKind.Relative))
+					uri = new Uri (nstring, UriKind.Relative);
+			}
+
+			return true;
+		}
+
+		static bool TryParse (string text, ref int index, out IList<Parameter> parameters)
+		{
+			string name, value;
+
+			parameters = null;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length)
+				return false;
+
+			if (text[index] != '(') {
+				if (index + 3 <= text.Length && text.Substring (index, 3) == "NIL") {
+					parameters = new List<Parameter> ();
+					index += 3;
+					return true;
+				}
+
+				return false;
+			}
+
+			index++;
+
+			if (index >= text.Length)
+				return false;
+
+			parameters = new List<Parameter> ();
+
+			do {
+				if (text[index] == ')')
+					break;
+
+				if (!TryParse (text, ref index, out name))
+					return false;
+
+				if (!TryParse (text, ref index, out value))
+					return false;
+
+				parameters.Add (new Parameter (name, value));
+			} while (index < text.Length);
+
+			if (index >= text.Length || text[index] != ')')
+				return false;
+
+			index++;
+
+			return true;
+		}
+
+		static bool TryParse (string text, ref int index, out ContentDisposition disposition)
+		{
+			IList<Parameter> parameters;
+			string value;
+
+			disposition = null;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length)
+				return false;
+
+			if (text[index] != '(') {
+				if (index + 3 <= text.Length && text.Substring (index, 3) == "NIL") {
+					index += 3;
+					return true;
+				}
+
+				return false;
+			}
+
+			index++;
+
+			if (!TryParse (text, ref index, out value))
+				return false;
+
+			if (!TryParse (text, ref index, out parameters))
+				return false;
+
+			if (index >= text.Length || text[index] != ')')
+				return false;
+
+			index++;
+
+			disposition = new ContentDisposition (value);
+
+			foreach (var param in parameters)
+				disposition.Parameters.Add (param);
+
+			return true;
+		}
+
+		static bool TryParse (string text, ref int index, bool multipart, out ContentType contentType)
+		{
+			IList<Parameter> parameters;
+			string type, subtype;
+
+			contentType = null;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length)
+				return false;
+
+			if (!multipart) {
+				if (!TryParse (text, ref index, out type))
+					return false;
+			} else {
+				type = "multipart";
+			}
+
+			if (!TryParse (text, ref index, out subtype))
+				return false;
+
+			if (!TryParse (text, ref index, out parameters))
+				return false;
+
+			contentType = new ContentType (type ?? "application", subtype ?? "octet-stream");
+
+			foreach (var param in parameters)
+				contentType.Parameters.Add (param);
+
+			return true;
+		}
+
+		static bool TryParse (string text, ref int index, string prefix, out IList<BodyPart> children)
+		{
+			BodyPart part;
+			string path;
+			int id = 1;
+
+			children = null;
+
+			if (index >= text.Length)
+				return false;
+
+			children = new List<BodyPart> ();
+
+			do {
+				if (text[index] != '(')
+					break;
+
+				path = prefix + id;
+
+				if (!TryParse (text, ref index, path, out part))
+					return false;
+
+				while (index < text.Length && text[index] == ' ')
+					index++;
+
+				children.Add (part);
+				id++;
+			} while (index < text.Length);
+
+			return index < text.Length;
+		}
+
+		static bool TryParse (string text, ref int index, string path, out BodyPart part)
+		{
+			ContentDisposition disposition;
+			ContentType contentType;
+			string[] array;
+			string nstring;
+			Uri location;
+			uint number;
+
+			part = null;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length || text[index] != '(') {
+				if (index + 3 <= text.Length && text.Substring (index, 3) == "NIL") {
+					index += 3;
+					return true;
+				}
+
+				return false;
+			}
+
+			index++;
+
+			if (index >= text.Length)
+				return false;
+
+			if (text[index] == '(') {
+				var prefix = path.Length > 0 ? path + "." : string.Empty;
+				var multipart = new BodyPartMultipart ();
+				IList<BodyPart> children;
+
+				if (!TryParse (text, ref index, prefix, out children))
+					return false;
+
+				foreach (var child in children)
+					multipart.BodyParts.Add (child);
+
+				if (!TryParse (text, ref index, true, out contentType))
+					return false;
+
+				multipart.ContentType = contentType;
+
+				if (!TryParse (text, ref index, out disposition))
+					return false;
+
+				multipart.ContentDisposition = disposition;
+
+				if (!TryParse (text, ref index, out array))
+					return false;
+
+				multipart.ContentLanguage = array;
+
+				if (!TryParse (text, ref index, out location))
+					return false;
+
+				multipart.ContentLocation = location;
+
+				part = multipart;
+			} else {
+				BodyPartMessage message = null;
+				BodyPartText txt = null;
+				BodyPartBasic basic;
+
+				if (!TryParse (text, ref index, false, out contentType))
+					return false;
+
+				if (contentType.IsMimeType ("message", "rfc822"))
+					basic = message = new BodyPartMessage ();
+				else if (contentType.IsMimeType ("text", "*"))
+					basic = txt = new BodyPartText ();
+				else
+					basic = new BodyPartBasic ();
+
+				basic.ContentType = contentType;
+
+				if (!TryParse (text, ref index, out nstring))
+					return false;
+
+				basic.ContentId = nstring;
+
+				if (!TryParse (text, ref index, out nstring))
+					return false;
+
+				basic.ContentDescription = nstring;
+
+				if (!TryParse (text, ref index, out nstring))
+					return false;
+
+				basic.ContentTransferEncoding = nstring;
+
+				if (!TryParse (text, ref index, out number))
+					return false;
+
+				basic.Octets = number;
+
+				if (!TryParse (text, ref index, out nstring))
+					return false;
+
+				basic.ContentMd5 = nstring;
+
+				if (!TryParse (text, ref index, out disposition))
+					return false;
+
+				basic.ContentDisposition = disposition;
+
+				if (!TryParse (text, ref index, out array))
+					return false;
+
+				basic.ContentLanguage = array;
+
+				if (!TryParse (text, ref index, out location))
+					return false;
+
+				basic.ContentLocation = location;
+
+				if (message != null) {
+					Envelope envelope;
+					BodyPart body;
+
+					if (!Envelope.TryParse (text, ref index, out envelope))
+						return false;
+
+					message.Envelope = envelope;
+
+					if (!TryParse (text, ref index, path, out body))
+						return false;
+
+					message.Body = body;
+
+					if (!TryParse (text, ref index, out number))
+						return false;
+
+					message.Lines = number;
+				} else if (txt != null) {
+					if (!TryParse (text, ref index, out number))
+						return false;
+
+					txt.Lines = number;
+				}
+
+				part = basic;
+			}
+
+			part.PartSpecifier = path;
+
+			if (index >= text.Length || text[index] != ')')
+				return false;
+
+			index++;
+
+			return true;
+		}
+
+		/// <summary>
+		/// Tries to parse the given text into a new <see cref="MailKit.BodyPart"/> instance.
+		/// </summary>
+		/// <remarks>
+		/// <para>Parses a body part from the specified text.</para>
+		/// <note type="note">This syntax, while similar to IMAP's BODYSTRUCTURE syntax, is not completely
+		/// compatible.</note>
+		/// </remarks>
+		/// <returns><c>true</c>, if the body part was successfully parsed, <c>false</c> otherwise.</returns>
+		/// <param name="text">The text to parse.</param>
+		/// <param name="part">The parsed body part.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="text"/> is <c>null</c>.
+		/// </exception>
+		public static bool TryParse (string text, out BodyPart part)
+		{
+			if (text == null)
+				throw new ArgumentNullException (nameof (text));
+
+			int index = 0;
+
+			return TryParse (text, ref index, string.Empty, out part) && index == text.Length;
+		}
+	}
+}

src/MailKit/BodyPartBasic.cs

@@ -0,0 +1,245 @@
+//
+// BodyPartBasic.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+
+using MimeKit;
+
+namespace MailKit {
+	/// <summary>
+	/// A basic message body part.
+	/// </summary>
+	/// <remarks>
+	/// Represents any message body part that is not a multipart,
+	/// message/rfc822 part, or a text part.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadBodyParts"/>
+	/// </example>
+	public class BodyPartBasic : BodyPart
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.BodyPartBasic"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="BodyPartBasic"/>.
+		/// </remarks>
+		public BodyPartBasic ()
+		{
+		}
+
+		/// <summary>
+		/// Gets the Content-Id of the body part, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the Content-Id of the body part, if available.
+		/// </remarks>
+		/// <value>The content identifier.</value>
+		public string ContentId {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the Content-Description of the body part, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the Content-Description of the body part, if available.
+		/// </remarks>
+		/// <value>The content description.</value>
+		public string ContentDescription {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the Content-Transfer-Encoding of the body part.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the Content-Transfer-Encoding of the body part.</para>
+		/// <para>Hint: Use the <a href="M_MimeKit_Utils_MimeUtils_TryParse_1.htm">MimeUtils.TryParse</a>
+		/// method to parse this value into a usable <see cref="MimeKit.ContentEncoding"/>.</para>
+		/// </remarks>
+		/// <value>The content transfer encoding.</value>
+		public string ContentTransferEncoding {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the size of the body part, in bytes.
+		/// </summary>
+		/// <remarks>
+		/// Gets the size of the body part, in bytes.
+		/// </remarks>
+		/// <value>The number of octets.</value>
+		public uint Octets {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the MD5 hash of the content, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the MD5 hash of the content, if available.
+		/// </remarks>
+		/// <value>The content md5.</value>
+		public string ContentMd5 {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the Content-Disposition of the body part, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the Content-Disposition of the body part, if available.</para>
+		/// <note type="note">The Content-Disposition value is only retrieved if the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is used when fetching
+		/// summary information from an <see cref="IMailFolder"/>.</note>
+		/// </remarks>
+		/// <value>The content disposition.</value>
+		public ContentDisposition ContentDisposition {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the Content-Language of the body part, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the Content-Language of the body part, if available.</para>
+		/// <para>The Content-Language value is only retrieved if the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is used when fetching
+		/// summary information from an <see cref="IMailFolder"/>.</para>
+		/// </remarks>
+		/// <value>The content language.</value>
+		public string[] ContentLanguage {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the Content-Location of the body part, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the Content-Location of the body part, if available.</para>
+		/// <para>The Content-Location value is only retrieved if the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is used when fetching
+		/// summary information from an <see cref="IMailFolder"/>.</para>
+		/// </remarks>
+		/// <value>The content location.</value>
+		public Uri ContentLocation {
+			get; set;
+		}
+
+		/// <summary>
+		/// Determines whether or not the body part is an attachment.
+		/// </summary>
+		/// <remarks>
+		/// <para>Determines whether or not the body part is an attachment based on the value of
+		/// the Content-Disposition.</para>
+		/// <note type="note">Since the value of the Content-Disposition header is needed, it
+		/// is necessary to include the <see cref="MessageSummaryItems.BodyStructure"/> flag when
+		/// fetching summary information from an <see cref="IMailFolder"/>.</note>
+		/// </remarks>
+		/// <value><c>true</c> if this part is an attachment; otherwise, <c>false</c>.</value>
+		public bool IsAttachment {
+			get { return ContentDisposition != null && ContentDisposition.IsAttachment; }
+		}
+
+		/// <summary>
+		/// Get the name of the file.
+		/// </summary>
+		/// <remarks>
+		/// <para>First checks for the "filename" parameter on the Content-Disposition header. If
+		/// that does not exist, then the "name" parameter on the Content-Type header is used.</para>
+		/// <note type="note">Since the value of the Content-Disposition header is needed, it is
+		/// necessary to include the <see cref="MessageSummaryItems.BodyStructure"/> flag when
+		/// fetching summary information from an <see cref="IMailFolder"/>.</note>
+		/// </remarks>
+		/// <value>The name of the file.</value>
+		public string FileName {
+			get {
+				string filename = null;
+
+				if (ContentDisposition != null)
+					filename = ContentDisposition.FileName;
+
+				if (filename == null)
+					filename = ContentType.Name;
+
+				return filename != null ? filename.Trim () : null;
+			}
+		}
+
+		/// <summary>
+		/// Dispatches to the specific visit method for this MIME body part.
+		/// </summary>
+		/// <remarks>
+		/// This default implementation for <see cref="MailKit.BodyPart"/> nodes
+		/// calls <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>. Override this
+		/// method to call into a more specific method on a derived visitor class
+		/// of the <see cref="MailKit.BodyPartVisitor"/> class. However, it should still
+		/// support unknown visitors by calling
+		/// <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>.
+		/// </remarks>
+		/// <param name="visitor">The visitor.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="visitor"/> is <c>null</c>.
+		/// </exception>
+		public override void Accept (BodyPartVisitor visitor)
+		{
+			if (visitor == null)
+				throw new ArgumentNullException (nameof (visitor));
+
+			visitor.VisitBodyPartBasic (this);
+		}
+
+		/// <summary>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </summary>
+		/// <remarks>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </remarks>
+		/// <param name="builder">The string builder.</param>
+		protected override void Encode (StringBuilder builder)
+		{
+			Encode (builder, ContentType);
+			builder.Append (' ');
+			Encode (builder, ContentId);
+			builder.Append (' ');
+			Encode (builder, ContentDescription);
+			builder.Append (' ');
+			Encode (builder, ContentTransferEncoding);
+			builder.Append (' ');
+			Encode (builder, Octets);
+			builder.Append (' ');
+			Encode (builder, ContentMd5);
+			builder.Append (' ');
+			Encode (builder, ContentDisposition);
+			builder.Append (' ');
+			Encode (builder, ContentLanguage);
+			builder.Append (' ');
+			Encode (builder, ContentLocation);
+		}
+	}
+}

src/MailKit/BodyPartCollection.cs

@@ -0,0 +1,276 @@
+//
+// BodyPartCollection.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Linq;
+using System.Collections;
+using System.Collections.Generic;
+
+using MimeKit.Utils;
+
+namespace MailKit {
+	/// <summary>
+	/// A <see cref="BodyPart"/> collection.
+	/// </summary>
+	/// <remarks>
+	/// A <see cref="BodyPart"/> collection.
+	/// </remarks>
+	public class BodyPartCollection : ICollection<BodyPart>
+	{
+		readonly List<BodyPart> collection = new List<BodyPart> ();
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.BodyPartCollection"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.BodyPartCollection"/>.
+		/// </remarks>
+		public BodyPartCollection ()
+		{
+		}
+
+		/// <summary>
+		/// Get the number of body parts in the collection.
+		/// </summary>
+		/// <remarks>
+		/// Gets the number of body parts in the collection.
+		/// </remarks>
+		/// <value>The count.</value>
+		public int Count {
+			get { return collection.Count; }
+		}
+
+		/// <summary>
+		/// Get whether or not this body part collection is read only.
+		/// </summary>
+		/// <remarks>
+		/// Gets whether or not this body part collection is read only.
+		/// </remarks>
+		/// <value><c>true</c> if this collection is read only; otherwise, <c>false</c>.</value>
+		public bool IsReadOnly {
+			get { return false; }
+		}
+
+		/// <summary>
+		/// Add the specified body part to the collection.
+		/// </summary>
+		/// <remarks>
+		/// Adds the specified body part to the collection.
+		/// </remarks>
+		/// <param name="part">The body part.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="part"/> is <c>null</c>.
+		/// </exception>
+		public void Add (BodyPart part)
+		{
+			if (part == null)
+				throw new ArgumentNullException (nameof (part));
+
+			collection.Add (part);
+		}
+
+		/// <summary>
+		/// Clears the body part collection.
+		/// </summary>
+		/// <remarks>
+		/// Removes all of the body parts from the collection.
+		/// </remarks>
+		public void Clear ()
+		{
+			collection.Clear ();
+		}
+
+		/// <summary>
+		/// Checks if the collection contains the specified body part.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether or not the collection contains the specified body part.
+		/// </remarks>
+		/// <returns><value>true</value> if the specified body part exists; otherwise <value>false</value>.</returns>
+		/// <param name="part">The body part.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="part"/> is <c>null</c>.
+		/// </exception>
+		public bool Contains (BodyPart part)
+		{
+			if (part == null)
+				throw new ArgumentNullException (nameof (part));
+
+			return collection.Contains (part);
+		}
+
+		/// <summary>
+		/// Copies all of the body parts in the collection to the specified array.
+		/// </summary>
+		/// <remarks>
+		/// Copies all of the body parts within the collection into the array,
+		/// starting at the specified array index.
+		/// </remarks>
+		/// <param name="array">The array.</param>
+		/// <param name="arrayIndex">The array index.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="array"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="arrayIndex"/> is out of range.
+		/// </exception>
+		public void CopyTo (BodyPart[] array, int arrayIndex)
+		{
+			if (array == null)
+				throw new ArgumentNullException (nameof (array));
+
+			if (arrayIndex < 0 || arrayIndex + Count > array.Length)
+				throw new ArgumentOutOfRangeException (nameof (arrayIndex));
+
+			collection.CopyTo (array, arrayIndex);
+		}
+
+		/// <summary>
+		/// Removes the specified body part.
+		/// </summary>
+		/// <remarks>
+		/// Removes the specified body part.
+		/// </remarks>
+		/// <returns><value>true</value> if the body part was removed; otherwise <value>false</value>.</returns>
+		/// <param name="part">The body part.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="part"/> is <c>null</c>.
+		/// </exception>
+		public bool Remove (BodyPart part)
+		{
+			if (part == null)
+				throw new ArgumentNullException (nameof (part));
+
+			return collection.Remove (part);
+		}
+
+		/// <summary>
+		/// Get the body part at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Gets the body part at the specified index.
+		/// </remarks>
+		/// <value>The body part at the specified index.</value>
+		/// <param name="index">The index.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public BodyPart this [int index] {
+			get {
+				if (index < 0 || index >= collection.Count)
+					throw new ArgumentOutOfRangeException (nameof (index));
+
+				return collection[index];
+			}
+		}
+
+		/// <summary>
+		/// Gets the index of the body part matching the specified URI.
+		/// </summary>
+		/// <remarks>
+		/// <para>Finds the index of the body part matching the specified URI, if it exists.</para>
+		/// <para>If the URI scheme is <c>"cid"</c>, then matching is performed based on the Content-Id header
+		/// values, otherwise the Content-Location headers are used. If the provided URI is absolute and a child
+		/// part's Content-Location is relative, then then the child part's Content-Location URI will be combined
+		/// with the value of its Content-Base header, if available, otherwise it will be combined with the
+		/// multipart/related part's Content-Base header in order to produce an absolute URI that can be
+		/// compared with the provided absolute URI.</para>
+		/// </remarks>
+		/// <returns>The index of the part matching the specified URI if found; otherwise <c>-1</c>.</returns>
+		/// <param name="uri">The URI of the body part.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="uri"/> is <c>null</c>.
+		/// </exception>
+		public int IndexOf (Uri uri)
+		{
+			if (uri == null)
+				throw new ArgumentNullException (nameof (uri));
+
+			bool cid = uri.IsAbsoluteUri && uri.Scheme.ToLowerInvariant () == "cid";
+
+			for (int index = 0; index < Count; index++) {
+				var bodyPart = this[index] as BodyPartBasic;
+
+				if (bodyPart == null)
+					continue;
+
+				if (uri.IsAbsoluteUri) {
+					if (cid) {
+						if (!string.IsNullOrEmpty (bodyPart.ContentId)) {
+							// Note: we might have a Content-Id in the form "<id@domain.com>", so attempt to decode it
+							var id = MimeUtils.EnumerateReferences (bodyPart.ContentId).FirstOrDefault () ?? bodyPart.ContentId;
+
+							if (id == uri.AbsolutePath)
+								return index;
+						}
+					} else if (bodyPart.ContentLocation != null) {
+						if (!bodyPart.ContentLocation.IsAbsoluteUri)
+							continue;
+
+						if (bodyPart.ContentLocation == uri)
+							return index;
+					}
+				} else if (bodyPart.ContentLocation == uri) {
+					return index;
+				}
+			}
+
+			return -1;
+		}
+
+		#region IEnumerable implementation
+
+		/// <summary>
+		/// Get the body part enumerator.
+		/// </summary>
+		/// <remarks>
+		/// Gets the body part enumerator.
+		/// </remarks>
+		/// <returns>The enumerator.</returns>
+		public IEnumerator<BodyPart> GetEnumerator ()
+		{
+			return collection.GetEnumerator ();
+		}
+
+		#endregion
+
+		#region IEnumerable implementation
+
+		/// <summary>
+		/// Get the body part enumerator.
+		/// </summary>
+		/// <remarks>
+		/// Gets the body part enumerator.
+		/// </remarks>
+		/// <returns>The enumerator.</returns>
+		IEnumerator IEnumerable.GetEnumerator ()
+		{
+			return GetEnumerator ();
+		}
+
+		#endregion
+	}
+}

src/MailKit/BodyPartMessage.cs

@@ -0,0 +1,124 @@
+//
+// BodyPartMessage.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+
+namespace MailKit {
+	/// <summary>
+	/// A message/rfc822 body part.
+	/// </summary>
+	/// <remarks>
+	/// Represents a message/rfc822 body part.
+	/// </remarks>
+	public class BodyPartMessage : BodyPartBasic
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.BodyPartMessage"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="BodyPartMessage"/>.
+		/// </remarks>
+		public BodyPartMessage ()
+		{
+		}
+
+		/// <summary>
+		/// Gets the envelope of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the envelope of the message, if available.
+		/// </remarks>
+		/// <value>The envelope.</value>
+		public Envelope Envelope {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the body structure of the message.
+		/// </summary>
+		/// <remarks>
+		/// Gets the body structure of the message.
+		/// </remarks>
+		/// <value>The body structure.</value>
+		public BodyPart Body {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the length of the message, in lines.
+		/// </summary>
+		/// <remarks>
+		/// Gets the length of the message, in lines.
+		/// </remarks>
+		/// <value>The number of lines.</value>
+		public uint Lines {
+			get; set;
+		}
+
+		/// <summary>
+		/// Dispatches to the specific visit method for this MIME body part.
+		/// </summary>
+		/// <remarks>
+		/// This default implementation for <see cref="MailKit.BodyPart"/> nodes
+		/// calls <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>. Override this
+		/// method to call into a more specific method on a derived visitor class
+		/// of the <see cref="MailKit.BodyPartVisitor"/> class. However, it should still
+		/// support unknown visitors by calling
+		/// <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>.
+		/// </remarks>
+		/// <param name="visitor">The visitor.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="visitor"/> is <c>null</c>.
+		/// </exception>
+		public override void Accept (BodyPartVisitor visitor)
+		{
+			if (visitor == null)
+				throw new ArgumentNullException (nameof (visitor));
+
+			visitor.VisitBodyPartMessage (this);
+		}
+
+		/// <summary>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </summary>
+		/// <remarks>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </remarks>
+		/// <param name="builder">The string builder.</param>
+		protected override void Encode (StringBuilder builder)
+		{
+			base.Encode (builder);
+
+			builder.Append (' ');
+			Encode (builder, Envelope);
+			builder.Append (' ');
+			Encode (builder, Body);
+			builder.Append (' ');
+			Encode (builder, Lines);
+		}
+	}
+}

src/MailKit/BodyPartMultipart.cs

@@ -0,0 +1,141 @@
+//
+// BodyPartMultipart.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+
+using MimeKit;
+
+namespace MailKit {
+	/// <summary>
+	/// A multipart body part.
+	/// </summary>
+	/// <remarks>
+	/// A multipart body part.
+	/// </remarks>
+	public class BodyPartMultipart : BodyPart
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.BodyPartMultipart"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="BodyPartMultipart"/>.
+		/// </remarks>
+		public BodyPartMultipart ()
+		{
+			BodyParts = new BodyPartCollection ();
+		}
+
+		/// <summary>
+		/// Gets the child body parts.
+		/// </summary>
+		/// <remarks>
+		/// Gets the child body parts.
+		/// </remarks>
+		/// <value>The child body parts.</value>
+		public BodyPartCollection BodyParts {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the Content-Disposition of the body part, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the Content-Disposition of the body part, if available.
+		/// </remarks>
+		/// <value>The content disposition.</value>
+		public ContentDisposition ContentDisposition {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the Content-Language of the body part, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the Content-Language of the body part, if available.
+		/// </remarks>
+		/// <value>The content language.</value>
+		public string[] ContentLanguage {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the Content-Location of the body part, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the Content-Location of the body part, if available.
+		/// </remarks>
+		/// <value>The content location.</value>
+		public Uri ContentLocation {
+			get; set;
+		}
+
+		/// <summary>
+		/// Dispatches to the specific visit method for this MIME body part.
+		/// </summary>
+		/// <remarks>
+		/// This default implementation for <see cref="MailKit.BodyPart"/> nodes
+		/// calls <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>. Override this
+		/// method to call into a more specific method on a derived visitor class
+		/// of the <see cref="MailKit.BodyPartVisitor"/> class. However, it should still
+		/// support unknown visitors by calling
+		/// <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>.
+		/// </remarks>
+		/// <param name="visitor">The visitor.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="visitor"/> is <c>null</c>.
+		/// </exception>
+		public override void Accept (BodyPartVisitor visitor)
+		{
+			if (visitor == null)
+				throw new ArgumentNullException (nameof (visitor));
+
+			visitor.VisitBodyPartMultipart (this);
+		}
+
+		/// <summary>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </summary>
+		/// <remarks>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </remarks>
+		/// <param name="builder">The string builder.</param>
+		protected override void Encode (StringBuilder builder)
+		{
+			Encode (builder, BodyParts);
+			builder.Append (' ');
+			Encode (builder, ContentType.MediaSubtype);
+			builder.Append (' ');
+			Encode (builder, ContentType.Parameters);
+			builder.Append (' ');
+			Encode (builder, ContentDisposition);
+			builder.Append (' ');
+			Encode (builder, ContentLanguage);
+			builder.Append (' ');
+			Encode (builder, ContentLocation);
+		}
+	}
+}

src/MailKit/BodyPartText.cs

@@ -0,0 +1,123 @@
+//
+// BodyPartText.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+
+namespace MailKit {
+	/// <summary>
+	/// A textual body part.
+	/// </summary>
+	/// <remarks>
+	/// Represents any body part with a media type of "text".
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadBodyParts"/>
+	/// </example>
+	public class BodyPartText : BodyPartBasic
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.BodyPartText"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="BodyPartText"/>.
+		/// </remarks>
+		public BodyPartText ()
+		{
+		}
+
+		/// <summary>
+		/// Gets whether or not this text part contains plain text.
+		/// </summary>
+		/// <remarks>
+		/// Checks whether or not the text part's Content-Type is text/plain.
+		/// </remarks>
+		/// <value><c>true</c> if the text is html; otherwise, <c>false</c>.</value>
+		public bool IsPlain {
+			get { return ContentType.IsMimeType ("text", "plain"); }
+		}
+
+		/// <summary>
+		/// Gets whether or not this text part contains HTML.
+		/// </summary>
+		/// <remarks>
+		/// Checks whether or not the text part's Content-Type is text/html.
+		/// </remarks>
+		/// <value><c>true</c> if the text is html; otherwise, <c>false</c>.</value>
+		public bool IsHtml {
+			get { return ContentType.IsMimeType ("text", "html"); }
+		}
+
+		/// <summary>
+		/// Gets the length of the text, in lines.
+		/// </summary>
+		/// <remarks>
+		/// Gets the length of the text, in lines.
+		/// </remarks>
+		/// <value>The number of lines.</value>
+		public uint Lines {
+			get; set;
+		}
+
+		/// <summary>
+		/// Dispatches to the specific visit method for this MIME body part.
+		/// </summary>
+		/// <remarks>
+		/// This default implementation for <see cref="MailKit.BodyPart"/> nodes
+		/// calls <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>. Override this
+		/// method to call into a more specific method on a derived visitor class
+		/// of the <see cref="MailKit.BodyPartVisitor"/> class. However, it should still
+		/// support unknown visitors by calling
+		/// <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>.
+		/// </remarks>
+		/// <param name="visitor">The visitor.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="visitor"/> is <c>null</c>.
+		/// </exception>
+		public override void Accept (BodyPartVisitor visitor)
+		{
+			if (visitor == null)
+				throw new ArgumentNullException (nameof (visitor));
+
+			visitor.VisitBodyPartText (this);
+		}
+
+		/// <summary>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </summary>
+		/// <remarks>
+		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
+		/// </remarks>
+		/// <param name="builder">The string builder.</param>
+		protected override void Encode (StringBuilder builder)
+		{
+			base.Encode (builder);
+
+			builder.Append (' ');
+			Encode (builder, Lines);
+		}
+	}
+}

src/MailKit/BodyPartVisitor.cs

@@ -0,0 +1,137 @@
+//
+// BodyPartVisitor.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit {
+	/// <summary>
+	/// Represents a visitor for a tree of MIME body parts.
+	/// </summary>
+	/// <remarks>
+	/// This class is designed to be inherited to create more specialized classes whose
+	/// functionality requires traversing, examining or copying a tree of MIME body parts.
+	/// </remarks>
+	public abstract class BodyPartVisitor
+	{
+		/// <summary>
+		/// Dispatches the entity to one of the more specialized visit methods in this class.
+		/// </summary>
+		/// <remarks>
+		/// Dispatches the entity to one of the more specialized visit methods in this class.
+		/// </remarks>
+		/// <param name="body">The MIME body part.</param>
+		public virtual void Visit (BodyPart body)
+		{
+			if (body != null)
+				body.Accept (this);
+		}
+
+		/// <summary>
+		/// Visit the abstract MIME body part.
+		/// </summary>
+		/// <remarks>
+		/// Visits the abstract MIME body part.
+		/// </remarks>
+		/// <param name="entity">The MIME body part.</param>
+		protected internal virtual void VisitBodyPart (BodyPart entity)
+		{
+		}
+
+		/// <summary>
+		/// Visit the basic MIME body part.
+		/// </summary>
+		/// <remarks>
+		/// Visits the basic MIME body part.
+		/// </remarks>
+		/// <param name="entity">The basic MIME body part.</param>
+		protected internal virtual void VisitBodyPartBasic (BodyPartBasic entity)
+		{
+			VisitBodyPart (entity);
+		}
+
+		/// <summary>
+		/// Visit the message contained within a message/rfc822 or message/news MIME entity.
+		/// </summary>
+		/// <remarks>
+		/// Visits the message contained within a message/rfc822 or message/news MIME entity.
+		/// </remarks>
+		/// <param name="message">The body part representing the message/rfc822 message.</param>
+		protected virtual void VisitMessage (BodyPart message)
+		{
+			if (message != null)
+				message.Accept (this);
+		}
+
+		/// <summary>
+		/// Visit the message/rfc822 or message/news MIME entity.
+		/// </summary>
+		/// <remarks>
+		/// Visits the message/rfc822 or message/news MIME entity.
+		/// </remarks>
+		/// <param name="entity">The message/rfc822 or message/news body part.</param>
+		protected internal virtual void VisitBodyPartMessage (BodyPartMessage entity)
+		{
+			VisitBodyPartBasic (entity);
+			VisitMessage (entity.Body);
+		}
+
+		/// <summary>
+		/// Visit the children of a <see cref="MailKit.BodyPartMultipart"/>.
+		/// </summary>
+		/// <remarks>
+		/// Visits the children of a <see cref="MailKit.BodyPartMultipart"/>.
+		/// </remarks>
+		/// <param name="multipart">The multipart.</param>
+		protected virtual void VisitChildren (BodyPartMultipart multipart)
+		{
+			for (int i = 0; i < multipart.BodyParts.Count; i++)
+				multipart.BodyParts[i].Accept (this);
+		}
+
+		/// <summary>
+		/// Visit the abstract multipart MIME entity.
+		/// </summary>
+		/// <remarks>
+		/// Visits the abstract multipart MIME entity.
+		/// </remarks>
+		/// <param name="multipart">The multipart body part.</param>
+		protected internal virtual void VisitBodyPartMultipart (BodyPartMultipart multipart)
+		{
+			VisitBodyPart (multipart);
+			VisitChildren (multipart);
+		}
+
+		/// <summary>
+		/// Visit the text-based MIME part entity.
+		/// </summary>
+		/// <remarks>
+		/// Visits the text-based MIME part entity.
+		/// </remarks>
+		/// <param name="entity">The text-based body part.</param>
+		protected internal virtual void VisitBodyPartText (BodyPartText entity)
+		{
+			VisitBodyPartBasic (entity);
+		}
+	}
+}

src/MailKit/CommandException.cs

@@ -0,0 +1,102 @@
+//
+// CommandException.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+#if SERIALIZABLE
+using System.Security;
+using System.Runtime.Serialization;
+#endif
+
+namespace MailKit {
+	/// <summary>
+	/// The exception that is thrown when there is a command error.
+	/// </summary>
+	/// <remarks>
+	/// A <see cref="CommandException"/> can be thrown by any of the various client
+	/// methods in MailKit. Unlike a <see cref="ProtocolException"/>, a
+	/// <see cref="CommandException"/> is typically non-fatal (meaning that it does
+	/// not force the client to disconnect).
+	/// </remarks>
+#if SERIALIZABLE
+	[Serializable]
+#endif
+	public abstract class CommandException : Exception
+	{
+#if SERIALIZABLE
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.CommandException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="CommandException"/>.
+		/// </remarks>
+		/// <param name="info">The serialization info.</param>
+		/// <param name="context">The streaming context.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="info"/> is <c>null</c>.
+		/// </exception>
+		[SecuritySafeCritical]
+		protected CommandException (SerializationInfo info, StreamingContext context) : base (info, context)
+		{
+		}
+#endif
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.CommandException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="CommandException"/>.
+		/// </remarks>
+		/// <param name="message">The error message.</param>
+		/// <param name="innerException">An inner exception.</param>
+		protected CommandException (string message, Exception innerException) : base (message, innerException)
+		{
+			HelpLink = "https://github.com/jstedfast/MailKit/blob/master/FAQ.md#ProtocolLog";
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.CommandException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="CommandException"/>.
+		/// </remarks>
+		/// <param name="message">The error message.</param>
+		protected CommandException (string message) : base (message)
+		{
+			HelpLink = "https://github.com/jstedfast/MailKit/blob/master/FAQ.md#ProtocolLog";
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.CommandException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="CommandException"/>.
+		/// </remarks>
+		protected CommandException ()
+		{
+			HelpLink = "https://github.com/jstedfast/MailKit/blob/master/FAQ.md#ProtocolLog";
+		}
+	}
+}

src/MailKit/CompressedStream.cs

@@ -0,0 +1,435 @@
+//
+// CompressedStream.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+using Org.BouncyCastle.Utilities.Zlib;
+
+namespace MailKit {
+	/// <summary>
+	/// A compressed stream.
+	/// </summary>
+	class CompressedStream : Stream
+	{
+		readonly ZStream zIn, zOut;
+		bool eos, disposed;
+
+		public CompressedStream (Stream innerStream)
+		{
+			InnerStream = innerStream;
+
+			zOut = new ZStream ();
+			zOut.deflateInit (5, true);
+			zOut.next_out = new byte[4096];
+
+			zIn = new ZStream ();
+			zIn.inflateInit (true);
+			zIn.next_in = new byte[4096];
+		}
+
+		/// <summary>
+		/// Gets the inner stream.
+		/// </summary>
+		/// <value>The inner stream.</value>
+		public Stream InnerStream {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets whether the stream supports reading.
+		/// </summary>
+		/// <value><c>true</c> if the stream supports reading; otherwise, <c>false</c>.</value>
+		public override bool CanRead {
+			get { return InnerStream.CanRead; }
+		}
+
+		/// <summary>
+		/// Gets whether the stream supports writing.
+		/// </summary>
+		/// <value><c>true</c> if the stream supports writing; otherwise, <c>false</c>.</value>
+		public override bool CanWrite {
+			get { return InnerStream.CanWrite; }
+		}
+
+		/// <summary>
+		/// Gets whether the stream supports seeking.
+		/// </summary>
+		/// <value><c>true</c> if the stream supports seeking; otherwise, <c>false</c>.</value>
+		public override bool CanSeek {
+			get { return false; }
+		}
+
+		/// <summary>
+		/// Gets whether the stream supports I/O timeouts.
+		/// </summary>
+		/// <value><c>true</c> if the stream supports I/O timeouts; otherwise, <c>false</c>.</value>
+		public override bool CanTimeout {
+			get { return InnerStream.CanTimeout; }
+		}
+
+		/// <summary>
+		/// Gets or sets a value, in miliseconds, that determines how long the stream will attempt to read before timing out.
+		/// </summary>
+		/// <returns>A value, in miliseconds, that determines how long the stream will attempt to read before timing out.</returns>
+		/// <value>The read timeout.</value>
+		public override int ReadTimeout {
+			get { return InnerStream.ReadTimeout; }
+			set { InnerStream.ReadTimeout = value; }
+		}
+
+		/// <summary>
+		/// Gets or sets a value, in miliseconds, that determines how long the stream will attempt to write before timing out.
+		/// </summary>
+		/// <returns>A value, in miliseconds, that determines how long the stream will attempt to write before timing out.</returns>
+		/// <value>The write timeout.</value>
+		public override int WriteTimeout {
+			get { return InnerStream.WriteTimeout; }
+			set { InnerStream.WriteTimeout = value; }
+		}
+
+		/// <summary>
+		/// Gets or sets the position within the current stream.
+		/// </summary>
+		/// <returns>The current position within the stream.</returns>
+		/// <value>The position of the stream.</value>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support seeking.
+		/// </exception>
+		public override long Position {
+			get { throw new NotSupportedException (); }
+			set { throw new NotSupportedException (); }
+		}
+
+		/// <summary>
+		/// Gets the length in bytes of the stream.
+		/// </summary>
+		/// <returns>A long value representing the length of the stream in bytes.</returns>
+		/// <value>The length of the stream.</value>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support seeking.
+		/// </exception>
+		public override long Length {
+			get { throw new NotSupportedException (); }
+		}
+
+		static void ValidateArguments (byte[] buffer, int offset, int count)
+		{
+			if (buffer == null)
+				throw new ArgumentNullException (nameof (buffer));
+
+			if (offset < 0 || offset > buffer.Length)
+				throw new ArgumentOutOfRangeException (nameof (offset));
+
+			if (count < 0 || count > (buffer.Length - offset))
+				throw new ArgumentOutOfRangeException (nameof (count));
+		}
+
+		void CheckDisposed ()
+		{
+			if (disposed)
+				throw new ObjectDisposedException (nameof (CompressedStream));
+		}
+
+		async Task<int> ReadAsync (byte[] buffer, int offset, int count, bool doAsync, CancellationToken cancellationToken)
+		{
+			CheckDisposed ();
+
+			ValidateArguments (buffer, offset, count);
+
+			if (count == 0)
+				return 0;
+
+			zIn.next_out = buffer;
+			zIn.next_out_index = offset;
+			zIn.avail_out = count;
+
+			do {
+				if (zIn.avail_in == 0 && !eos) {
+					cancellationToken.ThrowIfCancellationRequested ();
+
+					if (doAsync)
+						zIn.avail_in = await InnerStream.ReadAsync (zIn.next_in, 0, zIn.next_in.Length, cancellationToken).ConfigureAwait (false);
+					else
+						zIn.avail_in = InnerStream.Read (zIn.next_in, 0, zIn.next_in.Length);
+
+					eos = zIn.avail_in == 0;
+					zIn.next_in_index = 0;
+				}
+
+				int retval = zIn.inflate (JZlib.Z_FULL_FLUSH);
+
+				if (retval == JZlib.Z_STREAM_END)
+					break;
+
+				if (eos && retval == JZlib.Z_BUF_ERROR)
+					return 0;
+
+				if (retval != JZlib.Z_OK)
+					throw new IOException ("Error inflating: " + zIn.msg);
+			} while (zIn.avail_out == count);
+
+			return count - zIn.avail_out;
+		}
+
+		/// <summary>
+		/// Reads a sequence of bytes from the stream and advances the position
+		/// within the stream by the number of bytes read.
+		/// </summary>
+		/// <returns>The total number of bytes read into the buffer. This can be less than the number of bytes requested if that many
+		/// bytes are not currently available, or zero (0) if the end of the stream has been reached.</returns>
+		/// <param name="buffer">The buffer.</param>
+		/// <param name="offset">The buffer offset.</param>
+		/// <param name="count">The number of bytes to read.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override int Read (byte[] buffer, int offset, int count)
+		{
+			return ReadAsync (buffer, offset, count, false, CancellationToken.None).GetAwaiter ().GetResult ();
+		}
+
+		/// <summary>
+		/// Reads a sequence of bytes from the stream and advances the position
+		/// within the stream by the number of bytes read.
+		/// </summary>
+		/// <returns>The total number of bytes read into the buffer. This can be less than the number of bytes requested if that many
+		/// bytes are not currently available, or zero (0) if the end of the stream has been reached.</returns>
+		/// <param name="buffer">The buffer.</param>
+		/// <param name="offset">The buffer offset.</param>
+		/// <param name="count">The number of bytes to read.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override Task<int> ReadAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+		{
+			return ReadAsync (buffer, offset, count, true, cancellationToken);
+		}
+
+		async Task WriteAsync (byte[] buffer, int offset, int count, bool doAsync, CancellationToken cancellationToken)
+		{
+			CheckDisposed ();
+
+			ValidateArguments (buffer, offset, count);
+
+			if (count == 0)
+				return;
+
+			zOut.next_in = buffer;
+			zOut.next_in_index = offset;
+			zOut.avail_in = count;
+
+			do {
+				cancellationToken.ThrowIfCancellationRequested ();
+
+				zOut.avail_out = zOut.next_out.Length;
+				zOut.next_out_index = 0;
+
+				if (zOut.deflate (JZlib.Z_FULL_FLUSH) != JZlib.Z_OK)
+					throw new IOException ("Error deflating: " + zOut.msg);
+
+				if (doAsync)
+					await InnerStream.WriteAsync (zOut.next_out, 0, zOut.next_out.Length - zOut.avail_out, cancellationToken).ConfigureAwait (false);
+				else
+					InnerStream.Write (zOut.next_out, 0, zOut.next_out.Length - zOut.avail_out);
+			} while (zOut.avail_in > 0 || zOut.avail_out == 0);
+		}
+
+		/// <summary>
+		/// Writes a sequence of bytes to the stream and advances the current
+		/// position within this stream by the number of bytes written.
+		/// </summary>
+		/// <param name="buffer">The buffer to write.</param>
+		/// <param name="offset">The offset of the first byte to write.</param>
+		/// <param name="count">The number of bytes to write.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support writing.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override void Write (byte[] buffer, int offset, int count)
+		{
+			WriteAsync (buffer, offset, count, false, CancellationToken.None).GetAwaiter ().GetResult ();
+		}
+
+		/// <summary>
+		/// Writes a sequence of bytes to the stream and advances the current
+		/// position within this stream by the number of bytes written.
+		/// </summary>
+		/// <returns>A task that represents the asynchronous write operation.</returns>
+		/// <param name="buffer">The buffer to write.</param>
+		/// <param name="offset">The offset of the first byte to write.</param>
+		/// <param name="count">The number of bytes to write.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support writing.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override Task WriteAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+		{
+			return WriteAsync (buffer, offset, count, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Clears all output buffers for this stream and causes any buffered data to be written
+		/// to the underlying device.
+		/// </summary>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support writing.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override void Flush ()
+		{
+			CheckDisposed ();
+
+			InnerStream.Flush ();
+		}
+
+		/// <summary>
+		/// Clears all output buffers for this stream and causes any buffered data to be written
+		/// to the underlying device.
+		/// </summary>
+		/// <returns>A task that represents the asynchronous flush operation.</returns>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support writing.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override Task FlushAsync (CancellationToken cancellationToken)
+		{
+			CheckDisposed ();
+
+			return InnerStream.FlushAsync (cancellationToken);
+		}
+
+		/// <summary>
+		/// Sets the position within the current stream.
+		/// </summary>
+		/// <returns>The new position within the stream.</returns>
+		/// <param name="offset">The offset into the stream relative to the <paramref name="origin"/>.</param>
+		/// <param name="origin">The origin to seek from.</param>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support seeking.
+		/// </exception>
+		public override long Seek (long offset, SeekOrigin origin)
+		{
+			throw new NotSupportedException ();
+		}
+
+		/// <summary>
+		/// Sets the length of the stream.
+		/// </summary>
+		/// <param name="value">The desired length of the stream in bytes.</param>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support setting the length.
+		/// </exception>
+		public override void SetLength (long value)
+		{
+			throw new NotSupportedException ();
+		}
+
+		/// <summary>
+		/// Releases the unmanaged resources used by the <see cref="CompressedStream"/> and
+		/// optionally releases the managed resources.
+		/// </summary>
+		/// <param name="disposing"><c>true</c> to release both managed and unmanaged resources;
+		/// <c>false</c> to release only the unmanaged resources.</param>
+		protected override void Dispose (bool disposing)
+		{
+			if (disposing && !disposed) {
+				InnerStream.Dispose ();
+				disposed = true;
+				zOut.free ();
+				zIn.free ();
+			}
+
+			base.Dispose (disposing);
+		}
+	}
+}

src/MailKit/ConnectedEventArgs.cs

@@ -0,0 +1,88 @@
+//
+// ConnectedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+using MailKit.Security;
+
+namespace MailKit
+{
+	/// <summary>
+	/// Connected event arguments.
+	/// </summary>
+	/// <remarks>
+	/// When a <see cref="IMailService"/> is connected, it will emit a
+	/// <see cref=" IMailService.Connected"/> event.
+	/// </remarks>
+	public class ConnectedEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.ConnectedEventArgs"/> class.
+		/// </summary>
+		/// <param name="host">The name of the host that the client connected to.</param>
+		/// <param name="port">The port that the client connected to on the remote host.</param>
+		/// <param name="options">The SSL/TLS options that were used when connecting to the remote host.</param>
+		public ConnectedEventArgs (string host, int port, SecureSocketOptions options)
+		{
+			Options = options;
+			Host = host;
+			Port = port;
+		}
+
+		/// <summary>
+		/// Get the name of the remote host.
+		/// </summary>
+		/// <remarks>
+		/// Gets the name of the remote host.
+		/// </remarks>
+		/// <value>The host name of the server.</value>
+		public string Host {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the port.
+		/// </summary>
+		/// <remarks>
+		/// Gets the port.
+		/// </remarks>
+		/// <value>The port.</value>
+		public int Port {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the SSL/TLS options.
+		/// </summary>
+		/// <remarks>
+		/// Gets the SSL/TLS options.
+		/// </remarks>
+		/// <value>The SSL/TLS options.</value>
+		public SecureSocketOptions Options {
+			get; private set;
+		}
+	}
+}

src/MailKit/DeliveryStatusNotification.cs

@@ -0,0 +1,61 @@
+//
+// DeliveryStatusNotification.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// Delivery status notification types.
+	/// </summary>
+	/// <remarks>
+	/// A set of flags that may be bitwise-or'd together to specify
+	/// when a delivery status notification should be sent for a
+	/// particlar recipient.
+	/// </remarks>
+	[Flags]
+	public enum DeliveryStatusNotification {
+		/// <summary>
+		/// Never send delivery status notifications.
+		/// </summary>
+		Never   = 0,
+
+		/// <summary>
+		/// Send a notification on successful delivery to the recipient.
+		/// </summary>
+		Success = (1 << 0),
+
+		/// <summary>
+		/// Send a notification on failure to deliver to the recipient.
+		/// </summary>
+		Failure = (1 << 1),
+
+		/// <summary>
+		/// Send a notification when the delivery to the recipient has
+		/// been delayed for an unusual amount of time.
+		/// </summary>
+		Delay   = (1 << 2)
+	}
+}

src/MailKit/DeliveryStatusNotificationType.cs

@@ -0,0 +1,54 @@
+//
+// DeliveryStatusNotificationReturnType.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2019 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit.Net.Smtp
+{
+	/// <summary>
+	/// Delivery status notification type.
+	/// </summary>
+	/// <remarks>
+	/// The delivery status notification type specifies whether or not
+	/// the full message should be included in any failed DSN issued for
+	/// a message transmission as opposed to just the headers.
+	/// </remarks>
+	public enum DeliveryStatusNotificationType
+	{
+		/// <summary>
+		/// The return type is unspecified, allowing the server to choose.
+		/// </summary>
+		Unspecified,
+
+		/// <summary>
+		/// The full message should be included in any failed delivery status notification issued by the server.
+		/// </summary>
+		Full,
+
+		/// <summary>
+		/// Only the headers should be included in any failed delivery status notification issued by the server.
+		/// </summary>
+		HeadersOnly,
+	}
+}

src/MailKit/DisconnectedEventArgs.cs

@@ -0,0 +1,70 @@
+//
+// DisconnectedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using MailKit.Security;
+
+namespace MailKit
+{
+	/// <summary>
+	/// Disconnected event arguments.
+	/// </summary>
+	/// <remarks>
+	/// When a <see cref="IMailService"/> gets disconnected, it will emit a
+	/// <see cref=" IMailService.Disconnected"/> event.
+	/// </remarks>
+	public class DisconnectedEventArgs : ConnectedEventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.DisconnectedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Initializes a new instance of the <see cref="T:MailKit.DisconnectedEventArgs"/> class.
+		/// </remarks>
+		/// <param name="host">The name of the host that the client was connected to.</param>
+		/// <param name="port">The port that the client was connected to.</param>
+		/// <param name="options">The SSL/TLS options that were used by the client.</param>
+		/// <param name="requested">If <c>true</c>, the <see cref="IMailService"/> was disconnected via the
+		/// <see cref="IMailService.Disconnect(bool, System.Threading.CancellationToken)"/> method.</param>
+		public DisconnectedEventArgs (string host, int port, SecureSocketOptions options, bool requested) : base (host, port, options)
+		{
+			IsRequested = requested;
+		}
+
+		/// <summary>
+		/// Get whether or not the service was explicitly asked to disconnect.
+		/// </summary>
+		/// <remarks>
+		/// If the <see cref="IMailService"/> was disconnected via the
+		/// <see cref="IMailService.Disconnect(bool, System.Threading.CancellationToken)"/> method, then
+		/// the value of <see cref="IsRequested"/> will be <c>true</c>. If the connection was unexpectedly
+		/// dropped, then the value will be <c>false</c>.
+		/// </remarks>
+		/// <value><c>true</c> if the disconnect was explicitly requested; otherwise, <c>false</c>.</value>
+		public bool IsRequested {
+			get; private set;
+		}
+	}
+}

src/MailKit/DuplexStream.cs

@@ -0,0 +1,395 @@
+//
+// DuplexStream.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MailKit {
+	/// <summary>
+	/// A duplex stream.
+	/// </summary>
+	class DuplexStream : Stream
+	{
+		bool disposed;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.DuplexStream"/> class.
+		/// </summary>
+		/// <param name="istream">The stream to use for input.</param>
+		/// <param name="ostream">The stream to use for output.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="istream"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="ostream"/> is <c>null</c>.</para>
+		/// </exception>
+		public DuplexStream (Stream istream, Stream ostream)
+		{
+			if (istream == null)
+				throw new ArgumentNullException (nameof (istream));
+
+			if (ostream == null)
+				throw new ArgumentNullException (nameof (ostream));
+
+			InputStream = istream;
+			OutputStream = ostream;
+		}
+
+		/// <summary>
+		/// Gets the input stream.
+		/// </summary>
+		/// <value>The input stream.</value>
+		public Stream InputStream {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the output stream.
+		/// </summary>
+		/// <value>The output stream.</value>
+		public Stream OutputStream {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets whether the stream supports reading.
+		/// </summary>
+		/// <value><c>true</c> if the stream supports reading; otherwise, <c>false</c>.</value>
+		public override bool CanRead {
+			get { return true; }
+		}
+
+		/// <summary>
+		/// Gets whether the stream supports writing.
+		/// </summary>
+		/// <value><c>true</c> if the stream supports writing; otherwise, <c>false</c>.</value>
+		public override bool CanWrite {
+			get { return true; }
+		}
+
+		/// <summary>
+		/// Gets whether the stream supports seeking.
+		/// </summary>
+		/// <value><c>true</c> if the stream supports seeking; otherwise, <c>false</c>.</value>
+		public override bool CanSeek {
+			get { return false; }
+		}
+
+		/// <summary>
+		/// Gets whether the stream supports I/O timeouts.
+		/// </summary>
+		/// <value><c>true</c> if the stream supports I/O timeouts; otherwise, <c>false</c>.</value>
+		public override bool CanTimeout {
+			get { return InputStream.CanTimeout && OutputStream.CanTimeout; }
+		}
+
+		/// <summary>
+		/// Gets or sets a value, in miliseconds, that determines how long the stream will attempt to read before timing out.
+		/// </summary>
+		/// <returns>A value, in miliseconds, that determines how long the stream will attempt to read before timing out.</returns>
+		/// <value>The read timeout.</value>
+		public override int ReadTimeout {
+			get { return InputStream.ReadTimeout; }
+			set { InputStream.ReadTimeout = value; }
+		}
+
+		/// <summary>
+		/// Gets or sets a value, in miliseconds, that determines how long the stream will attempt to write before timing out.
+		/// </summary>
+		/// <returns>A value, in miliseconds, that determines how long the stream will attempt to write before timing out.</returns>
+		/// <value>The write timeout.</value>
+		public override int WriteTimeout {
+			get { return OutputStream.WriteTimeout; }
+			set { OutputStream.WriteTimeout = value; }
+		}
+
+		/// <summary>
+		/// Gets or sets the position within the current stream.
+		/// </summary>
+		/// <returns>The current position within the stream.</returns>
+		/// <value>The position of the stream.</value>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support seeking.
+		/// </exception>
+		public override long Position {
+			get { throw new NotSupportedException (); }
+			set { throw new NotSupportedException (); }
+		}
+
+		/// <summary>
+		/// Gets the length in bytes of the stream.
+		/// </summary>
+		/// <returns>A long value representing the length of the stream in bytes.</returns>
+		/// <value>The length of the stream.</value>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support seeking.
+		/// </exception>
+		public override long Length {
+			get { throw new NotSupportedException (); }
+		}
+
+		static void ValidateArguments (byte[] buffer, int offset, int count)
+		{
+			if (buffer == null)
+				throw new ArgumentNullException (nameof (buffer));
+
+			if (offset < 0 || offset > buffer.Length)
+				throw new ArgumentOutOfRangeException (nameof (offset));
+
+			if (count < 0 || count > (buffer.Length - offset))
+				throw new ArgumentOutOfRangeException (nameof (count));
+		}
+
+		void CheckDisposed ()
+		{
+			if (disposed)
+				throw new ObjectDisposedException (nameof (DuplexStream));
+		}
+
+		/// <summary>
+		/// Reads a sequence of bytes from the stream and advances the position
+		/// within the stream by the number of bytes read.
+		/// </summary>
+		/// <returns>The total number of bytes read into the buffer. This can be less than the number of bytes requested if that many
+		/// bytes are not currently available, or zero (0) if the end of the stream has been reached.</returns>
+		/// <param name="buffer">The buffer.</param>
+		/// <param name="offset">The buffer offset.</param>
+		/// <param name="count">The number of bytes to read.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override int Read (byte[] buffer, int offset, int count)
+		{
+			CheckDisposed ();
+
+			ValidateArguments (buffer, offset, count);
+
+			return InputStream.Read (buffer, offset, count);
+		}
+
+		/// <summary>
+		/// Reads a sequence of bytes from the stream and advances the position
+		/// within the stream by the number of bytes read.
+		/// </summary>
+		/// <returns>The total number of bytes read into the buffer. This can be less than the number of bytes requested if that many
+		/// bytes are not currently available, or zero (0) if the end of the stream has been reached.</returns>
+		/// <param name="buffer">The buffer.</param>
+		/// <param name="offset">The buffer offset.</param>
+		/// <param name="count">The number of bytes to read.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override Task<int> ReadAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+		{
+			CheckDisposed ();
+
+			ValidateArguments (buffer, offset, count);
+
+			return InputStream.ReadAsync (buffer, offset, count, cancellationToken);
+		}
+
+		/// <summary>
+		/// Writes a sequence of bytes to the stream and advances the current
+		/// position within this stream by the number of bytes written.
+		/// </summary>
+		/// <param name="buffer">The buffer to write.</param>
+		/// <param name="offset">The offset of the first byte to write.</param>
+		/// <param name="count">The number of bytes to write.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support writing.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override void Write (byte[] buffer, int offset, int count)
+		{
+			CheckDisposed ();
+
+			ValidateArguments (buffer, offset, count);
+
+			OutputStream.Write (buffer, offset, count);
+		}
+
+		/// <summary>
+		/// Writes a sequence of bytes to the stream and advances the current
+		/// position within this stream by the number of bytes written.
+		/// </summary>
+		/// <returns>A task that represents the asynchronous write operation.</returns>
+		/// <param name="buffer">The buffer to write.</param>
+		/// <param name="offset">The offset of the first byte to write.</param>
+		/// <param name="count">The number of bytes to write.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support writing.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override Task WriteAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+		{
+			CheckDisposed ();
+
+			ValidateArguments (buffer, offset, count);
+
+			return OutputStream.WriteAsync (buffer, offset, count, cancellationToken);
+		}
+
+		/// <summary>
+		/// Clears all output buffers for this stream and causes any buffered data to be written
+		/// to the underlying device.
+		/// </summary>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support writing.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override void Flush ()
+		{
+			CheckDisposed ();
+
+			OutputStream.Flush ();
+		}
+
+		/// <summary>
+		/// Clears all output buffers for this stream and causes any buffered data to be written
+		/// to the underlying device.
+		/// </summary>
+		/// <returns>A task that represents the asynchronous flush operation.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The stream has been disposed.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support writing.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override Task FlushAsync (CancellationToken cancellationToken)
+		{
+			CheckDisposed ();
+
+			return OutputStream.FlushAsync (cancellationToken);
+		}
+
+		/// <summary>
+		/// Sets the position within the current stream.
+		/// </summary>
+		/// <returns>The new position within the stream.</returns>
+		/// <param name="offset">The offset into the stream relative to the <paramref name="origin"/>.</param>
+		/// <param name="origin">The origin to seek from.</param>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support seeking.
+		/// </exception>
+		public override long Seek (long offset, SeekOrigin origin)
+		{
+			throw new NotSupportedException ();
+		}
+
+		/// <summary>
+		/// Sets the length of the stream.
+		/// </summary>
+		/// <param name="value">The desired length of the stream in bytes.</param>
+		/// <exception cref="System.NotSupportedException">
+		/// The stream does not support setting the length.
+		/// </exception>
+		public override void SetLength (long value)
+		{
+			throw new NotSupportedException ();
+		}
+
+		/// <summary>
+		/// Releases the unmanaged resources used by the <see cref="DuplexStream"/> and
+		/// optionally releases the managed resources.
+		/// </summary>
+		/// <param name="disposing"><c>true</c> to release both managed and unmanaged resources;
+		/// <c>false</c> to release only the unmanaged resources.</param>
+		protected override void Dispose (bool disposing)
+		{
+			if (disposing && !disposed) {
+				OutputStream.Dispose ();
+				InputStream.Dispose ();
+				disposed = true;
+			}
+
+			base.Dispose (disposing);
+		}
+	}
+}

src/MailKit/Envelope.cs

@@ -0,0 +1,592 @@
+//
+// Envelope.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+using System.Linq;
+using System.Collections.Generic;
+
+using MimeKit;
+using MimeKit.Utils;
+
+namespace MailKit {
+	/// <summary>
+	/// A message envelope containing a brief summary of the message.
+	/// </summary>
+	/// <remarks>
+	/// The envelope of a message contains information such as the
+	/// date the message was sent, the subject of the message,
+	/// the sender of the message, who the message was sent to,
+	/// which message(s) the message may be in reply to,
+	/// and the message id.
+	/// </remarks>
+	public class Envelope
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Envelope"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="Envelope"/>.
+		/// </remarks>
+		public Envelope ()
+		{
+			From = new InternetAddressList ();
+			Sender = new InternetAddressList ();
+			ReplyTo = new InternetAddressList ();
+			To = new InternetAddressList ();
+			Cc = new InternetAddressList ();
+			Bcc = new InternetAddressList ();
+		}
+
+		/// <summary>
+		/// Gets the address(es) that the message is from.
+		/// </summary>
+		/// <remarks>
+		/// Gets the address(es) that the message is from.
+		/// </remarks>
+		/// <value>The address(es) that the message is from.</value>
+		public InternetAddressList From {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the actual sender(s) of the message.
+		/// </summary>
+		/// <remarks>
+		/// The senders may differ from the addresses in <see cref="From"/> if
+		/// the message was sent by someone on behalf of someone else.
+		/// </remarks>
+		/// <value>The actual sender(s) of the message.</value>
+		public InternetAddressList Sender {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the address(es) that replies should be sent to.
+		/// </summary>
+		/// <remarks>
+		/// The senders of the message may prefer that replies are sent
+		/// somewhere other than the address they used to send the message.
+		/// </remarks>
+		/// <value>The address(es) that replies should be sent to.</value>
+		public InternetAddressList ReplyTo {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the list of addresses that the message was sent to.
+		/// </summary>
+		/// <remarks>
+		/// Gets the list of addresses that the message was sent to.
+		/// </remarks>
+		/// <value>The address(es) that the message was sent to.</value>
+		public InternetAddressList To {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the list of addresses that the message was carbon-copied to.
+		/// </summary>
+		/// <remarks>
+		/// Gets the list of addresses that the message was carbon-copied to.
+		/// </remarks>
+		/// <value>The address(es) that the message was carbon-copied to.</value>
+		public InternetAddressList Cc {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the list of addresses that the message was blind-carbon-copied to.
+		/// </summary>
+		/// <remarks>
+		/// Gets the list of addresses that the message was blind-carbon-copied to.
+		/// </remarks>
+		/// <value>The address(es) that the message was carbon-copied to.</value>
+		public InternetAddressList Bcc {
+			get; private set;
+		}
+
+		/// <summary>
+		/// The Message-Id that the message is replying to.
+		/// </summary>
+		/// <remarks>
+		/// The Message-Id that the message is replying to.
+		/// </remarks>
+		/// <value>The Message-Id that the message is replying to.</value>
+		public string InReplyTo {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the date that the message was sent on, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the date that the message was sent on, if available.
+		/// </remarks>
+		/// <value>The date the message was sent.</value>
+		public DateTimeOffset? Date {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the ID of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the ID of the message, if available.
+		/// </remarks>
+		/// <value>The message identifier.</value>
+		public string MessageId {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the subject of the message.
+		/// </summary>
+		/// <remarks>
+		/// Gets the subject of the message.
+		/// </remarks>
+		/// <value>The subject.</value>
+		public string Subject {
+			get; set;
+		}
+
+		static void EncodeMailbox (StringBuilder builder, MailboxAddress mailbox)
+		{
+			builder.Append ('(');
+
+			if (mailbox.Name != null)
+				builder.AppendFormat ("{0} ", MimeUtils.Quote (mailbox.Name));
+			else
+				builder.Append ("NIL ");
+
+			if (mailbox.Route.Count != 0)
+				builder.AppendFormat ("\"{0}\" ", mailbox.Route);
+			else
+				builder.Append ("NIL ");
+
+			int at = mailbox.Address.LastIndexOf ('@');
+
+			if (at >= 0) {
+				var domain = mailbox.Address.Substring (at + 1);
+				var user = mailbox.Address.Substring (0, at);
+
+				builder.AppendFormat ("{0} {1}", MimeUtils.Quote (user), MimeUtils.Quote (domain));
+			} else {
+				builder.AppendFormat ("{0} \"localhost\"", MimeUtils.Quote (mailbox.Address));
+			}
+
+			builder.Append (')');
+		}
+
+		static void EncodeInternetAddressListAddresses (StringBuilder builder, InternetAddressList addresses)
+		{
+			foreach (var addr in addresses) {
+				var mailbox = addr as MailboxAddress;
+				var group = addr as GroupAddress;
+
+				if (mailbox != null)
+					EncodeMailbox (builder, mailbox);
+				else if (group != null)
+					EncodeGroup (builder, group);
+			}
+		}
+
+		static void EncodeGroup (StringBuilder builder, GroupAddress group)
+		{
+			builder.AppendFormat ("(NIL NIL {0} NIL)", MimeUtils.Quote (group.Name));
+			EncodeInternetAddressListAddresses (builder, group.Members);
+			builder.Append ("(NIL NIL NIL NIL)");
+		}
+
+		static void EncodeAddressList (StringBuilder builder, InternetAddressList list)
+		{
+			builder.Append ('(');
+			EncodeInternetAddressListAddresses (builder, list);
+			builder.Append (')');
+		}
+
+		internal void Encode (StringBuilder builder)
+		{
+			builder.Append ('(');
+
+			if (Date.HasValue)
+				builder.AppendFormat ("\"{0}\" ", DateUtils.FormatDate (Date.Value));
+			else
+				builder.Append ("NIL ");
+
+			if (Subject != null)
+				builder.AppendFormat ("{0} ", MimeUtils.Quote (Subject));
+			else
+				builder.Append ("NIL ");
+
+			if (From.Count > 0) {
+				EncodeAddressList (builder, From);
+				builder.Append (' ');
+			} else {
+				builder.Append ("NIL ");
+			}
+
+			if (Sender.Count > 0) {
+				EncodeAddressList (builder, Sender);
+				builder.Append (' ');
+			} else {
+				builder.Append ("NIL ");
+			}
+
+			if (ReplyTo.Count > 0) {
+				EncodeAddressList (builder, ReplyTo);
+				builder.Append (' ');
+			} else {
+				builder.Append ("NIL ");
+			}
+
+			if (To.Count > 0) {
+				EncodeAddressList (builder, To);
+				builder.Append (' ');
+			} else {
+				builder.Append ("NIL ");
+			}
+
+			if (Cc.Count > 0) {
+				EncodeAddressList (builder, Cc);
+				builder.Append (' ');
+			} else {
+				builder.Append ("NIL ");
+			}
+
+			if (Bcc.Count > 0) {
+				EncodeAddressList (builder, Bcc);
+				builder.Append (' ');
+			} else {
+				builder.Append ("NIL ");
+			}
+
+			if (InReplyTo != null) {
+				if (InReplyTo.Length > 1 && InReplyTo[0] != '<' && InReplyTo[InReplyTo.Length - 1] != '>')
+					builder.AppendFormat ("{0} ", MimeUtils.Quote ('<' + InReplyTo + '>'));
+				else
+					builder.AppendFormat ("{0} ", MimeUtils.Quote (InReplyTo));
+			} else
+				builder.Append ("NIL ");
+
+			if (MessageId != null) {
+				if (MessageId.Length > 1 && MessageId[0] != '<' && MessageId[MessageId.Length - 1] != '>')
+					builder.AppendFormat ("{0}", MimeUtils.Quote ('<' + MessageId + '>'));
+				else
+					builder.AppendFormat ("{0}", MimeUtils.Quote (MessageId));
+			} else
+				builder.Append ("NIL");
+
+			builder.Append (')');
+		}
+
+		/// <summary>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.Envelope"/>.
+		/// </summary>
+		/// <remarks>
+		/// <para>The returned string can be parsed by <see cref="TryParse(string,out Envelope)"/>.</para>
+		/// <note type="warning">The syntax of the string returned, while similar to IMAP's ENVELOPE syntax,
+		/// is not completely compatible.</note>
+		/// </remarks>
+		/// <returns>A <see cref="System.String"/> that represents the current <see cref="MailKit.Envelope"/>.</returns>
+		public override string ToString ()
+		{
+			var builder = new StringBuilder ();
+
+			Encode (builder);
+
+			return builder.ToString ();
+		}
+
+		static bool TryParse (string text, ref int index, out string nstring)
+		{
+			nstring = null;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length)
+				return false;
+
+			if (text[index] != '"') {
+				if (index + 3 <= text.Length && text.Substring (index, 3) == "NIL") {
+					index += 3;
+					return true;
+				}
+
+				return false;
+			}
+
+			var token = new StringBuilder ();
+			bool escaped = false;
+
+			index++;
+
+			while (index < text.Length) {
+				if (text[index] == '"' && !escaped)
+					break;
+
+				if (escaped || text[index] != '\\') {
+					token.Append (text[index]);
+					escaped = false;
+				} else {
+					escaped = true;
+				}
+
+				index++;
+			}
+
+			if (index >= text.Length)
+				return false;
+
+			nstring = token.ToString ();
+
+			index++;
+
+			return true;
+		}
+
+		static bool TryParse (string text, ref int index, out InternetAddress addr)
+		{
+			string name, route, user, domain;
+			DomainList domains;
+
+			addr = null;
+
+			if (text[index] != '(')
+				return false;
+
+			index++;
+
+			if (!TryParse (text, ref index, out name))
+				return false;
+
+			if (!TryParse (text, ref index, out route))
+				return false;
+
+			if (!TryParse (text, ref index, out user))
+				return false;
+
+			if (!TryParse (text, ref index, out domain))
+				return false;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length || text[index] != ')')
+				return false;
+
+			index++;
+
+			if (domain != null) {
+				var address = user + "@" + domain;
+
+				if (route != null && DomainList.TryParse (route, out domains))
+					addr = new MailboxAddress (name, domains, address);
+				else
+					addr = new MailboxAddress (name, address);
+			} else if (user != null) {
+				addr = new GroupAddress (user);
+			}
+
+			return true;
+		}
+
+		static bool TryParse (string text, ref int index, out InternetAddressList list)
+		{
+			list = null;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length)
+				return false;
+
+			if (text[index] != '(') {
+				if (index + 3 <= text.Length && text.Substring (index, 3) == "NIL") {
+					list = new InternetAddressList ();
+					index += 3;
+					return true;
+				}
+
+				return false;
+			}
+
+			index++;
+
+			if (index >= text.Length)
+				return false;
+
+			list = new InternetAddressList ();
+			var stack = new List<InternetAddressList> ();
+			int sp = 0;
+
+			stack.Add (list);
+
+			do {
+				if (text[index] == ')')
+					break;
+
+				if (!TryParse (text, ref index, out InternetAddress addr))
+					return false;
+
+				if (addr != null) {
+					var group = addr as GroupAddress;
+
+					stack[sp].Add (addr);
+
+					if (group != null) {
+						stack.Add (group.Members);
+						sp++;
+					}
+				} else if (sp > 0) {
+					stack.RemoveAt (sp);
+					sp--;
+				}
+
+				while (index < text.Length && text[index] == ' ')
+					index++;
+			} while (index < text.Length);
+
+			// Note: technically, we should check that sp == 0 as well, since all groups should
+			// be popped off the stack, but in the interest of being liberal in what we accept,
+			// we'll ignore that.
+			if (index >= text.Length)
+				return false;
+
+			index++;
+
+			return true;
+		}
+
+		internal static bool TryParse (string text, ref int index, out Envelope envelope)
+		{
+			InternetAddressList from, sender, replyto, to, cc, bcc;
+			string inreplyto, messageid, subject, nstring;
+			DateTimeOffset? date = null;
+
+			envelope = null;
+
+			while (index < text.Length && text[index] == ' ')
+				index++;
+
+			if (index >= text.Length || text[index] != '(') {
+				if (index + 3 <= text.Length && text.Substring (index, 3) == "NIL") {
+					index += 3;
+					return true;
+				}
+
+				return false;
+			}
+
+			index++;
+
+			if (!TryParse (text, ref index, out nstring))
+				return false;
+
+			if (nstring != null) {
+				DateTimeOffset value;
+
+				if (!DateUtils.TryParse (nstring, out value))
+					return false;
+
+				date = value;
+			}
+
+			if (!TryParse (text, ref index, out subject))
+				return false;
+
+			if (!TryParse (text, ref index, out from))
+				return false;
+
+			if (!TryParse (text, ref index, out sender))
+				return false;
+
+			if (!TryParse (text, ref index, out replyto))
+				return false;
+
+			if (!TryParse (text, ref index, out to))
+				return false;
+
+			if (!TryParse (text, ref index, out cc))
+				return false;
+
+			if (!TryParse (text, ref index, out bcc))
+				return false;
+
+			if (!TryParse (text, ref index, out inreplyto))
+				return false;
+
+			if (!TryParse (text, ref index, out messageid))
+				return false;
+
+			if (index >= text.Length || text[index] != ')')
+				return false;
+
+			index++;
+
+			envelope = new Envelope {
+				Date = date,
+				Subject = subject,
+				From = from,
+				Sender = sender,
+				ReplyTo = replyto,
+				To = to,
+				Cc = cc,
+				Bcc = bcc,
+				InReplyTo = inreplyto != null ? MimeUtils.EnumerateReferences (inreplyto).FirstOrDefault () ?? inreplyto : null,
+				MessageId = messageid != null ? MimeUtils.EnumerateReferences (messageid).FirstOrDefault () ?? messageid : null
+			};
+
+			return true;
+		}
+
+		/// <summary>
+		/// Tries to parse the given text into a new <see cref="MailKit.Envelope"/> instance.
+		/// </summary>
+		/// <remarks>
+		/// <para>Parses an Envelope value from the specified text.</para>
+		/// <note type="warning">This syntax, while similar to IMAP's ENVELOPE syntax, is not
+		/// completely compatible.</note>
+		/// </remarks>
+		/// <returns><c>true</c>, if the envelope was successfully parsed, <c>false</c> otherwise.</returns>
+		/// <param name="text">The text to parse.</param>
+		/// <param name="envelope">The parsed envelope.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="text"/> is <c>null</c>.
+		/// </exception>
+		public static bool TryParse (string text, out Envelope envelope)
+		{
+			if (text == null)
+				throw new ArgumentNullException (nameof (text));
+
+			int index = 0;
+
+			return TryParse (text, ref index, out envelope) && index == text.Length;
+		}
+	}
+}
+	

src/MailKit/FolderAccess.cs

@@ -0,0 +1,53 @@
+//
+// FolderMode.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit {
+	/// <summary>
+	/// A folder access mode.
+	/// </summary>
+	/// <remarks>
+	/// A folder access mode.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadMessages"/>
+	/// </example>
+	public enum FolderAccess {
+		/// <summary>
+		/// The folder is not open.
+		/// </summary>
+		None,
+
+		/// <summary>
+		/// The folder is read-only.
+		/// </summary>
+		ReadOnly,
+
+		/// <summary>
+		/// The folder is read/write.
+		/// </summary>
+		ReadWrite
+	}
+}

src/MailKit/FolderAttributes.cs

@@ -0,0 +1,135 @@
+//
+// FolderAttributes.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// Folder attributes as used by <see cref="IMailFolder.Attributes"/>.
+	/// </summary>
+	/// <remarks>
+	/// Folder attributes as used by <see cref="IMailFolder.Attributes"/>.
+	/// </remarks>
+	[Flags]
+	public enum FolderAttributes {
+		/// <summary>
+		/// The folder does not have any attributes.
+		/// </summary>
+		None          = 0,
+
+		/// <summary>
+		/// It is not possible for any subfolders to exist under the folder.
+		/// </summary>
+		NoInferiors   = (1 << 0),
+
+		/// <summary>
+		/// It is not possible to select the folder.
+		/// </summary>
+		NoSelect      = (1 << 1),
+
+		/// <summary>
+		/// The folder has been marked as possibly containing new messages
+		/// since the folder was last selected.
+		/// </summary>
+		Marked        = (1 << 2),
+
+		/// <summary>
+		/// The folder does not contain any new messages since the folder
+		/// was last selected.
+		/// </summary>
+		Unmarked      = (1 << 3),
+
+		/// <summary>
+		/// The folder does not exist, but is simply a place-holder.
+		/// </summary>
+		NonExistent   = (1 << 4),
+
+		/// <summary>
+		/// The folder is subscribed.
+		/// </summary>
+		Subscribed    = (1 << 5),
+
+		/// <summary>
+		/// The folder is remote.
+		/// </summary>
+		Remote        = (1 << 6),
+
+		/// <summary>
+		/// The folder has subfolders.
+		/// </summary>
+		HasChildren   = (1 << 7),
+
+		/// <summary>
+		/// The folder does not have any subfolders.
+		/// </summary>
+		HasNoChildren = (1 << 8),
+
+		/// <summary>
+		/// The folder is a special "All" folder containing an aggregate of all messages.
+		/// </summary>
+		All           = (1 << 9),
+
+		/// <summary>
+		/// The folder is a special "Archive" folder.
+		/// </summary>
+		Archive       = (1 << 10),
+
+		/// <summary>
+		/// The folder is the special "Drafts" folder.
+		/// </summary>
+		Drafts        = (1 << 11),
+
+		/// <summary>
+		/// The folder is the special "Flagged" folder.
+		/// </summary>
+		Flagged       = (1 << 12),
+
+		/// <summary>
+		/// The folder is the special "Important" folder.
+		/// </summary>
+		Important     = (1 << 13),
+
+		/// <summary>
+		/// The folder is the special "Inbox" folder.
+		/// </summary>
+		Inbox         = (1 << 14),
+
+		/// <summary>
+		/// The folder is the special "Junk" folder.
+		/// </summary>
+		Junk          = (1 << 15),
+
+		/// <summary>
+		/// The folder is the special "Sent" folder.
+		/// </summary>
+		Sent          = (1 << 16),
+
+		/// <summary>
+		/// The folder is the special "Trash" folder.
+		/// </summary>
+		Trash         = (1 << 17),
+	}
+}

src/MailKit/FolderCreatedEventArgs.cs

@@ -0,0 +1,67 @@
+//
+// FolderCreatedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args used when a <see cref="IMailFolder"/> is created.
+	/// </summary>
+	/// <remarks>
+	/// Event args used when a <see cref="IMailFolder"/> is created.
+	/// </remarks>
+	public class FolderCreatedEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.FolderCreatedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderCreatedEventArgs"/>.
+		/// </remarks>
+		/// <param name="folder">The newly created folder.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="folder"/> is <c>null</c>.
+		/// </exception>
+		public FolderCreatedEventArgs (IMailFolder folder)
+		{
+			if (folder == null)
+				throw new ArgumentNullException (nameof (folder));
+
+			Folder = folder;
+		}
+
+		/// <summary>
+		/// Get the folder that was just created.
+		/// </summary>
+		/// <remarks>
+		/// Gets the folder that was just created.
+		/// </remarks>
+		/// <value>The folder.</value>
+		public IMailFolder Folder {
+			get; private set;
+		}
+	}
+}

src/MailKit/FolderFeature.cs

@@ -0,0 +1,82 @@
+//
+// FolderFeature.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit
+{
+	/// <summary>
+	/// An optional feature that an <see cref="IMailFolder"/> may support.
+	/// </summary>
+	/// <remarks>
+	/// An optional feature that an <see cref="IMailFolder"/> may support.
+	/// </remarks>
+	public enum FolderFeature
+	{
+		/// <summary>
+		/// Indicates that the folder supports access rights.
+		/// </summary>
+		AccessRights,
+
+		/// <summary>
+		/// Indicates that the folder allows arbitrary annotations to be set on a message.
+		/// </summary>
+		Annotations,
+
+		/// <summary>
+		/// Indicates that the folder allows arbitrary metadata to be set.
+		/// </summary>
+		Metadata,
+
+		/// <summary>
+		/// Indicates that the folder uses modification sequences for every state change of a message.
+		/// </summary>
+		ModSequences,
+
+		/// <summary>
+		/// Indicates that the folder supports quick resynchronization when opening.
+		/// </summary>
+		QuickResync,
+
+		/// <summary>
+		/// Indicates that the folder supports quotas.
+		/// </summary>
+		Quotas,
+
+		/// <summary>
+		/// Indicates that the folder supports sorting messages.
+		/// </summary>
+		Sorting,
+
+		/// <summary>
+		/// Indicates that the folder supports threading messages.
+		/// </summary>
+		Threading,
+
+		/// <summary>
+		/// Indicates that the folder supports the use of UTF-8.
+		/// </summary>
+		UTF8,
+	}
+}

src/MailKit/FolderNamespace.cs

@@ -0,0 +1,74 @@
+//
+// FolderNamespace.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// A folder namespace.
+	/// </summary>
+	/// <remarks>
+	/// A folder namespace.
+	/// </remarks>
+	public class FolderNamespace
+	{
+		/// <summary>
+		/// The directory separator for this folder namespace.
+		/// </summary>
+		/// <remarks>
+		/// The directory separator for this folder namespace.
+		/// </remarks>
+		public readonly char DirectorySeparator;
+
+		/// <summary>
+		/// The base path for this folder namespace.
+		/// </summary>
+		/// <remarks>
+		/// The base path for this folder namespace.
+		/// </remarks>
+		public readonly string Path;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.FolderNamespace"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new folder namespace.
+		/// </remarks>
+		/// <param name="directorySeparator">The directory separator.</param>
+		/// <param name="path">The folder path.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="path"/> is <c>null</c>.
+		/// </exception>
+		public FolderNamespace (char directorySeparator, string path)
+		{
+			if (path == null)
+				throw new ArgumentNullException (nameof (path));
+
+			DirectorySeparator = directorySeparator;
+			Path = path;
+		}
+	}
+}

src/MailKit/FolderNamespaceCollection.cs

@@ -0,0 +1,235 @@
+//
+// FolderNamespaceCollection.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+using System.Collections;
+using System.Collections.Generic;
+
+using MimeKit.Utils;
+
+namespace MailKit {
+	/// <summary>
+	/// A read-only collection of folder namespaces.
+	/// </summary>
+	/// <remarks>
+	/// A read-only collection of folder namespaces.
+	/// </remarks>
+	public class FolderNamespaceCollection : IEnumerable<FolderNamespace>
+	{
+		readonly List<FolderNamespace> namespaces;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderNamespaceCollection"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderNamespaceCollection"/>.
+		/// </remarks>
+		public FolderNamespaceCollection ()
+		{
+			namespaces = new List<FolderNamespace> ();
+		}
+
+		#region ICollection implementation
+
+		/// <summary>
+		/// Gets the number of folder namespaces contained in the collection.
+		/// </summary>
+		/// <remarks>
+		/// Gets the number of folder namespaces contained in the collection.
+		/// </remarks>
+		/// <value>The count.</value>
+		public int Count {
+			get { return namespaces.Count; }
+		}
+
+		/// <summary>
+		/// Adds the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Adds the specified namespace.
+		/// </remarks>
+		/// <param name="namespace">The namespace to add.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		public void Add (FolderNamespace @namespace)
+		{
+			if (@namespace == null)
+				throw new ArgumentNullException (nameof (@namespace));
+
+			namespaces.Add (@namespace);
+		}
+
+		/// <summary>
+		/// Removes all namespaces from the collection.
+		/// </summary>
+		/// <remarks>
+		/// Removes all namespaces from the collection.
+		/// </remarks>
+		public void Clear ()
+		{
+			namespaces.Clear ();
+		}
+
+		/// <summary>
+		/// Checks if the collection contains the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Checks if the collection contains the specified namespace.
+		/// </remarks>
+		/// <returns><value>true</value> if the specified namespace exists;
+		/// otherwise <value>false</value>.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		public bool Contains (FolderNamespace @namespace)
+		{
+			if (@namespace == null)
+				throw new ArgumentNullException (nameof (@namespace));
+
+			return namespaces.Contains (@namespace);
+		}
+
+		/// <summary>
+		/// Removes the first occurance of the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Removes the first occurance of the specified namespace.
+		/// </remarks>
+		/// <returns><value>true</value> if the frst occurance of the specified
+		/// namespace was removed; otherwise <value>false</value>.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		public bool Remove (FolderNamespace @namespace)
+		{
+			if (@namespace == null)
+				throw new ArgumentNullException (nameof (@namespace));
+
+			return namespaces.Remove (@namespace);
+		}
+
+		/// <summary>
+		/// Gets the <see cref="MailKit.FolderNamespace"/> at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Gets the <see cref="MailKit.FolderNamespace"/> at the specified index.
+		/// </remarks>
+		/// <value>The folder namespace at the specified index.</value>
+		/// <param name="index">The index.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="value"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public FolderNamespace this [int index] {
+			get {
+				if (index < 0 || index >= namespaces.Count)
+					throw new ArgumentOutOfRangeException (nameof (index));
+
+				return namespaces[index];
+			}
+			set {
+				if (index < 0 || index >= namespaces.Count)
+					throw new ArgumentOutOfRangeException (nameof (index));
+
+				if (value == null)
+					throw new ArgumentNullException (nameof (value));
+
+				namespaces[index] = value;
+			}
+		}
+
+		#endregion
+
+		#region IEnumerable implementation
+
+		/// <summary>
+		/// Gets the enumerator.
+		/// </summary>
+		/// <remarks>
+		/// Gets the enumerator.
+		/// </remarks>
+		/// <returns>The enumerator.</returns>
+		public IEnumerator<FolderNamespace> GetEnumerator ()
+		{
+			return namespaces.GetEnumerator ();
+		}
+
+		#endregion
+
+		#region IEnumerable implementation
+
+		/// <summary>
+		/// Gets the enumerator.
+		/// </summary>
+		/// <remarks>
+		/// Gets the enumerator.
+		/// </remarks>
+		/// <returns>The enumerator.</returns>
+		IEnumerator IEnumerable.GetEnumerator ()
+		{
+			return namespaces.GetEnumerator ();
+		}
+
+		#endregion
+
+		static bool Escape (char directorySeparator)
+		{
+			return directorySeparator == '\\' || directorySeparator == '"';
+		}
+
+		/// <summary>
+		/// Returns a <see cref="T:System.String"/> that represents the current <see cref="T:MailKit.FolderNamespaceCollection"/>.
+		/// </summary>
+		/// <remarks>
+		/// Returns a <see cref="T:System.String"/> that represents the current <see cref="T:MailKit.FolderNamespaceCollection"/>.
+		/// </remarks>
+		/// <returns>A <see cref="T:System.String"/> that represents the current <see cref="T:MailKit.FolderNamespaceCollection"/>.</returns>
+		public override string ToString ()
+		{
+			var builder = new StringBuilder ();
+
+			builder.Append ('(');
+			for (int i = 0; i < namespaces.Count; i++) {
+				builder.Append ("(\"");
+				if (Escape (namespaces[i].DirectorySeparator))
+					builder.Append ('\\');
+				builder.Append (namespaces[i].DirectorySeparator);
+				builder.Append ("\" ");
+				builder.Append (MimeUtils.Quote (namespaces[i].Path));
+				builder.Append (")");
+			}
+			builder.Append (')');
+
+			return builder.ToString ();
+		}
+	}
+}

src/MailKit/FolderNotFoundException.cs

@@ -0,0 +1,149 @@
+//
+// FolderNotFoundException.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+#if SERIALIZABLE
+using System.Security;
+using System.Runtime.Serialization;
+#endif
+
+namespace MailKit {
+	/// <summary>
+	/// The exception that is thrown when a folder could not be found.
+	/// </summary>
+	/// <remarks>
+	/// This exception is thrown by <see cref="IMailFolder.GetSubfolder(string,System.Threading.CancellationToken)"/>.
+	/// </remarks>
+#if SERIALIZABLE
+	[Serializable]
+#endif
+	public class FolderNotFoundException : Exception
+	{
+#if SERIALIZABLE
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderNotFoundException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Deserializes a <see cref="FolderNotFoundException"/>.
+		/// </remarks>
+		/// <param name="info">The serialization info.</param>
+		/// <param name="context">The streaming context.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="info"/> is <c>null</c>.
+		/// </exception>
+		protected FolderNotFoundException (SerializationInfo info, StreamingContext context) : base (info, context)
+		{
+			FolderName = info.GetString ("FolderName");
+		}
+#endif
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderNotFoundException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderNotFoundException"/>.
+		/// </remarks>
+		/// <param name="message">The error message.</param>
+		/// <param name="folderName">The name of the folder.</param>
+		/// <param name="innerException">The inner exception.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="folderName"/> is <c>null</c>.
+		/// </exception>
+		public FolderNotFoundException (string message, string folderName, Exception innerException) : base (message, innerException)
+		{
+			if (folderName == null)
+				throw new ArgumentNullException (nameof (folderName));
+
+			FolderName = folderName;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderNotFoundException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderNotFoundException"/>.
+		/// </remarks>
+		/// <param name="message">The error message.</param>
+		/// <param name="folderName">The name of the folder.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="folderName"/> is <c>null</c>.
+		/// </exception>
+		public FolderNotFoundException (string message, string folderName) : base (message)
+		{
+			if (folderName == null)
+				throw new ArgumentNullException (nameof (folderName));
+
+			FolderName = folderName;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderNotFoundException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderNotFoundException"/>.
+		/// </remarks>
+		/// <param name="folderName">The name of the folder.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="folderName"/> is <c>null</c>.
+		/// </exception>
+		public FolderNotFoundException (string folderName) : this ("The requested folder could not be found.", folderName)
+		{
+		}
+
+		/// <summary>
+		/// Gets the name of the folder that could not be found.
+		/// </summary>
+		/// <remarks>
+		/// Gets the name of the folder that could not be found.
+		/// </remarks>
+		/// <value>The name of the folder.</value>
+		public string FolderName {
+			get; private set;
+		}
+
+#if SERIALIZABLE
+		/// <summary>
+		/// When overridden in a derived class, sets the <see cref="System.Runtime.Serialization.SerializationInfo"/>
+		/// with information about the exception.
+		/// </summary>
+		/// <remarks>
+		/// Serializes the state of the <see cref="FolderNotFoundException"/>.
+		/// </remarks>
+		/// <param name="info">The serialization info.</param>
+		/// <param name="context">The streaming context.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="info"/> is <c>null</c>.
+		/// </exception>
+		[SecurityCritical]
+		public override void GetObjectData (SerializationInfo info, StreamingContext context)
+		{
+			base.GetObjectData (info, context);
+
+			info.AddValue ("FolderName", FolderName);
+		}
+#endif
+	}
+}

src/MailKit/FolderNotOpenException.cs

@@ -0,0 +1,184 @@
+//
+// FolderNotOpenException.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+#if SERIALIZABLE
+using System.Runtime.Serialization;
+#endif
+
+namespace MailKit {
+	/// <summary>
+	/// The exception that is thrown when a folder is not open.
+	/// </summary>
+	/// <remarks>
+	/// This exception is thrown when an operation on a folder could not be completed
+	/// due to the folder being in a closed state. For example, the
+	/// <see cref="IMailFolder.GetMessage(UniqueId,System.Threading.CancellationToken, ITransferProgress)"/>
+	/// method will throw a <see cref="FolderNotOpenException"/> if the folder is not
+	/// current open.
+	/// </remarks>
+#if SERIALIZABLE
+	[Serializable]
+#endif
+	public class FolderNotOpenException : InvalidOperationException
+	{
+#if SERIALIZABLE
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderNotOpenException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Deserializes a <see cref="FolderNotOpenException"/>.
+		/// </remarks>
+		/// <param name="info">The serialization info.</param>
+		/// <param name="context">The streaming context.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="info"/> is <c>null</c>.
+		/// </exception>
+		protected FolderNotOpenException (SerializationInfo info, StreamingContext context) : base (info, context)
+		{
+			var value = info.GetString ("FolderAccess");
+			FolderAccess access;
+
+			if (!Enum.TryParse (value, out access))
+				FolderAccess = FolderAccess.ReadOnly;
+			else
+				FolderAccess = access;
+
+			FolderName = info.GetString ("FolderName");
+		}
+#endif
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderNotOpenException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderNotOpenException"/>.
+		/// </remarks>
+		/// <param name="folderName">The folder name.</param>
+		/// <param name="access">The minimum folder access required by the operation.</param>
+		/// <param name="message">The error message.</param>
+		/// <param name="innerException">The inner exception.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="folderName"/> is <c>null</c>.
+		/// </exception>
+		public FolderNotOpenException (string folderName, FolderAccess access, string message, Exception innerException) : base (message, innerException)
+		{
+			if (folderName == null)
+				throw new ArgumentNullException (nameof (folderName));
+
+			FolderName = folderName;
+			FolderAccess = access;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderNotOpenException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderNotOpenException"/>.
+		/// </remarks>
+		/// <param name="folderName">The folder name.</param>
+		/// <param name="access">The minimum folder access required by the operation.</param>
+		/// <param name="message">The error message.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="folderName"/> is <c>null</c>.
+		/// </exception>
+		public FolderNotOpenException (string folderName, FolderAccess access, string message) : base (message)
+		{
+			if (folderName == null)
+				throw new ArgumentNullException (nameof (folderName));
+
+			FolderName = folderName;
+			FolderAccess = access;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderNotOpenException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderNotOpenException"/>.
+		/// </remarks>
+		/// <param name="folderName">The folder name.</param>
+		/// <param name="access">The minimum folder access required by the operation.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="folderName"/> is <c>null</c>.
+		/// </exception>
+		public FolderNotOpenException (string folderName, FolderAccess access) : this (folderName, access, GetDefaultMessage (access))
+		{
+		}
+
+		/// <summary>
+		/// Get the name of the folder.
+		/// </summary>
+		/// <remarks>
+		/// Gets the name of the folder.
+		/// </remarks>
+		/// <value>The name of the folder.</value>
+		public string FolderName {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the minimum folder access required by the operation.
+		/// </summary>
+		/// <remarks>
+		/// Gets the minimum folder access required by the operation.
+		/// </remarks>
+		/// <value>The minimum required folder access.</value>
+		public FolderAccess FolderAccess {
+			get; private set;
+		}
+
+		static string GetDefaultMessage (FolderAccess access)
+		{
+			if (access == FolderAccess.ReadWrite)
+				return "The folder is not currently open in read-write mode.";
+
+			return "The folder is not currently open.";
+		}
+
+#if SERIALIZABLE
+		/// <summary>
+		/// When overridden in a derived class, sets the <see cref="System.Runtime.Serialization.SerializationInfo"/>
+		/// with information about the exception.
+		/// </summary>
+		/// <remarks>
+		/// Serializes the state of the <see cref="FolderNotOpenException"/>.
+		/// </remarks>
+		/// <param name="info">The serialization info.</param>
+		/// <param name="context">The streaming context.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="info"/> is <c>null</c>.
+		/// </exception>
+		public override void GetObjectData (SerializationInfo info, StreamingContext context)
+		{
+			base.GetObjectData (info, context);
+
+			info.AddValue ("FolderAccess", FolderAccess.ToString ());
+			info.AddValue ("FolderName", FolderName);
+		}
+#endif
+	}
+}

src/MailKit/FolderQuota.cs

@@ -0,0 +1,124 @@
+//
+// FolderQuota.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// A folder quota.
+	/// </summary>
+	/// <remarks>
+	/// A <see cref="FolderQuota"/> is returned by <see cref="IMailFolder.GetQuota(System.Threading.CancellationToken)"/>.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+	/// </example>
+	public class FolderQuota
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.FolderQuota"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderQuota"/> with the specified root.
+		/// </remarks>
+		/// <param name="quotaRoot">The quota root.</param>
+		public FolderQuota (IMailFolder quotaRoot)
+		{
+			QuotaRoot = quotaRoot;
+		}
+
+		/// <summary>
+		/// Get the quota root.
+		/// </summary>
+		/// <remarks>
+		/// Gets the quota root. If the quota root is <c>null</c>, then
+		/// it suggests that the folder does not have a quota.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The quota root.</value>
+		public IMailFolder QuotaRoot {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get or set the message limit.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the message limit.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The message limit.</value>
+		public uint? MessageLimit {
+			get; set;
+		}
+
+		/// <summary>
+		/// Get or set the storage limit, in kilobytes.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the storage limit, in kilobytes.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The storage limit, in kilobytes.</value>
+		public uint? StorageLimit {
+			get; set;
+		}
+
+		/// <summary>
+		/// Get or set the current message count.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the current message count.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The current message count.</value>
+		public uint? CurrentMessageCount {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets or sets the size of the current storage, in kilobytes.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the size of the current storage, in kilobytes.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The size of the current storage, in kilobytes.</value>
+		public uint? CurrentStorageSize {
+			get; set;
+		}
+	}
+}

src/MailKit/FolderRenamedEventArgs.cs

@@ -0,0 +1,85 @@
+//
+// FolderRenamedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args used when a <see cref="IMailFolder"/> is renamed.
+	/// </summary>
+	/// <remarks>
+	/// Event args used when a <see cref="IMailFolder"/> is renamed.
+	/// </remarks>
+	public class FolderRenamedEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.FolderRenamedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="FolderRenamedEventArgs"/>.
+		/// </remarks>
+		/// <param name="oldName">The old name of the folder.</param>
+		/// <param name="newName">The new name of the folder.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="oldName"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="newName"/> is <c>null</c>.</para>
+		/// </exception>
+		public FolderRenamedEventArgs (string oldName, string newName)
+		{
+			if (oldName == null)
+				throw new ArgumentNullException (nameof (oldName));
+
+			if (newName == null)
+				throw new ArgumentNullException (nameof (newName));
+
+			OldName = oldName;
+			NewName = newName;
+		}
+
+		/// <summary>
+		/// The old name of the folder.
+		/// </summary>
+		/// <remarks>
+		/// The old name of the folder.
+		/// </remarks>
+		/// <value>The old name.</value>
+		public string OldName {
+			get; private set;
+		}
+
+		/// <summary>
+		/// The new name of the folder.
+		/// </summary>
+		/// <remarks>
+		/// The new name of the folder.
+		/// </remarks>
+		/// <value>The new name.</value>
+		public string NewName {
+			get; private set;
+		}
+	}
+}

src/MailKit/IMailSpool.cs

@@ -0,0 +1,511 @@
+//
+// IMailSpool.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+using MimeKit;
+
+namespace MailKit {
+	/// <summary>
+	/// An interface for retreiving messages from a spool.
+	/// </summary>
+	/// <remarks>
+	/// An interface for retreiving messages from a spool.
+	/// </remarks>
+	public interface IMailSpool : IMailService, IEnumerable<MimeMessage>
+	{
+		/// <summary>
+		/// Get the number of messages available in the message spool.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the number of messages available in the message spool.</para>
+		/// <para>Once authenticated, the <see cref="Count"/> property will be set
+		/// to the number of available messages in the spool.</para>
+		/// </remarks>
+		/// <value>The message count.</value>
+		int Count { get; }
+
+		/// <summary>
+		/// Get whether or not the service supports referencing messages by UIDs.
+		/// </summary>
+		/// <remarks>
+		/// <para>Not all servers support referencing messages by UID, so this property should
+		/// be checked before using <see cref="GetMessageUid(int, CancellationToken)"/>
+		/// and <see cref="GetMessageUids(CancellationToken)"/>.</para>
+		/// <para>If the server does not support UIDs, then all methods that take UID arguments
+		/// along with <see cref="GetMessageUid(int, CancellationToken)"/> and
+		/// <see cref="GetMessageUids(CancellationToken)"/> will fail.</para>
+		/// </remarks>
+		/// <value><c>true</c> if supports uids; otherwise, <c>false</c>.</value>
+		bool SupportsUids { get; }
+
+		/// <summary>
+		/// Get the message count.
+		/// </summary>
+		/// <remarks>
+		/// Gets the message count.
+		/// </remarks>
+		/// <returns>The message count.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		int GetMessageCount (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the UID of the message at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Not all servers support UIDs, so you should first check
+		/// the <see cref="SupportsUids"/> property.
+		/// </remarks>
+		/// <returns>The message UID.</returns>
+		/// <param name="index">The message index.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		string GetMessageUid (int index, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get the UID of the message at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Not all servers support UIDs, so you should first check
+		/// the <see cref="SupportsUids"/> property.
+		/// </remarks>
+		/// <returns>The message UID.</returns>
+		/// <param name="index">The message index.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<string> GetMessageUidAsync (int index, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the full list of available message UIDs.
+		/// </summary>
+		/// <remarks>
+		/// Not all servers support UIDs, so you should first check
+		/// the <see cref="SupportsUids"/> property.
+		/// </remarks>
+		/// <returns>The message UIDs.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		IList<string> GetMessageUids (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get the full list of available message UIDs.
+		/// </summary>
+		/// <remarks>
+		/// Not all servers support UIDs, so you should first check
+		/// the <see cref="SupportsUids"/> property.
+		/// </remarks>
+		/// <returns>The message UIDs.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<IList<string>> GetMessageUidsAsync (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the size of the specified message, in bytes.
+		/// </summary>
+		/// <remarks>
+		/// Gets the size of the specified message, in bytes.
+		/// </remarks>
+		/// <returns>The message size, in bytes.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		int GetMessageSize (int index, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get the size of the specified message, in bytes.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the size of the specified message, in bytes.
+		/// </remarks>
+		/// <returns>The message size, in bytes.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<int> GetMessageSizeAsync (int index, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the sizes for all available messages, in bytes.
+		/// </summary>
+		/// <remarks>
+		/// Gets the sizes for all available messages, in bytes.
+		/// </remarks>
+		/// <returns>The message sizes, in bytes.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		IList<int> GetMessageSizes (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get the sizes for all available messages, in bytes.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the sizes for all available messages, in bytes.
+		/// </remarks>
+		/// <returns>The message sizes, in bytes.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<IList<int>> GetMessageSizesAsync (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the headers for the specified message.
+		/// </summary>
+		/// <remarks>
+		/// Gets the headers for the specified message.
+		/// </remarks>
+		/// <returns>The message headers.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		HeaderList GetMessageHeaders (int index, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get the headers for the specified message.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the headers for the specified message.
+		/// </remarks>
+		/// <returns>The message headers.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<HeaderList> GetMessageHeadersAsync (int index, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the headers for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// Gets the headers for the specified messages.
+		/// </remarks>
+		/// <returns>The headers for the specified messages.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		IList<HeaderList> GetMessageHeaders (IList<int> indexes, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get the headers for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the headers for the specified messages.
+		/// </remarks>
+		/// <returns>The headers for the specified messages.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<IList<HeaderList>> GetMessageHeadersAsync (IList<int> indexes, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the headers of the messages within the specified range.
+		/// </summary>
+		/// <remarks>
+		/// Gets the headers of the messages within the specified range.
+		/// </remarks>
+		/// <returns>The headers of the messages within the specified range.</returns>
+		/// <param name="startIndex">The index of the first message to get.</param>
+		/// <param name="count">The number of messages to get.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		IList<HeaderList> GetMessageHeaders (int startIndex, int count, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the headers of the messages within the specified range.
+		/// </summary>
+		/// <remarks>
+		/// Gets the headers of the messages within the specified range.
+		/// </remarks>
+		/// <returns>The headers of the messages within the specified range.</returns>
+		/// <param name="startIndex">The index of the first message to get.</param>
+		/// <param name="count">The number of messages to get.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<IList<HeaderList>> GetMessageHeadersAsync (int startIndex, int count, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the message at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Gets the message at the specified index.
+		/// </remarks>
+		/// <returns>The message.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		MimeMessage GetMessage (int index, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the message at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the message at the specified index.
+		/// </remarks>
+		/// <returns>The message.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task<MimeMessage> GetMessageAsync (int index, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the messages at the specified indexes.
+		/// </summary>
+		/// <remarks>
+		/// Gets the messages at the specified indexes.
+		/// </remarks>
+		/// <returns>The messages.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		IList<MimeMessage> GetMessages (IList<int> indexes, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the messages at the specified indexes.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the messages at the specified indexes.
+		/// </remarks>
+		/// <returns>The messages.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task<IList<MimeMessage>> GetMessagesAsync (IList<int> indexes, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the messages within the specified range.
+		/// </summary>
+		/// <remarks>
+		/// Gets the messages within the specified range.
+		/// </remarks>
+		/// <returns>The messages.</returns>
+		/// <param name="startIndex">The index of the first message to get.</param>
+		/// <param name="count">The number of messages to get.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		IList<MimeMessage> GetMessages (int startIndex, int count, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the messages within the specified range.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the messages within the specified range.
+		/// </remarks>
+		/// <returns>The messages.</returns>
+		/// <param name="startIndex">The index of the first message to get.</param>
+		/// <param name="count">The number of messages to get.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task<IList<MimeMessage>> GetMessagesAsync (int startIndex, int count, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the message or header stream at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Gets the message or header stream at the specified index.
+		/// </remarks>
+		/// <returns>The message or header stream.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="headersOnly"><c>true</c> if only the headers should be retrieved; otherwise, <c>false</c>.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Stream GetStream (int index, bool headersOnly = false, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the message or header stream at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the message or header stream at the specified index.
+		/// </remarks>
+		/// <returns>The message or header stream.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="headersOnly"><c>true</c> if only the headers should be retrieved; otherwise, <c>false</c>.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task<Stream> GetStreamAsync (int index, bool headersOnly = false, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the message or header streams at the specified index.
+		/// </summary>
+		/// <remarks>
+		/// Gets the message or header streams at the specified index.
+		/// </remarks>
+		/// <returns>The message or header streams.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="headersOnly"><c>true</c> if only the headers should be retrieved; otherwise, <c>false</c>.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		IList<Stream> GetStreams (IList<int> indexes, bool headersOnly = false, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the message or header streams at the specified indexes.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the message or header streams at the specified indexes.
+		/// </remarks>
+		/// <returns>The message or header streams.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="headersOnly"><c>true</c> if only the headers should be retrieved; otherwise, <c>false</c>.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task<IList<Stream>> GetStreamsAsync (IList<int> indexes, bool headersOnly = false, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the message or header streams within the specified range.
+		/// </summary>
+		/// <remarks>
+		/// Gets the message or header streams within the specified range.
+		/// </remarks>
+		/// <returns>The message or header streams.</returns>
+		/// <param name="startIndex">The index of the first stream to get.</param>
+		/// <param name="count">The number of streams to get.</param>
+		/// <param name="headersOnly"><c>true</c> if only the headers should be retrieved; otherwise, <c>false</c>.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		IList<Stream> GetStreams (int startIndex, int count, bool headersOnly = false, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the message or header streams within the specified range.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the message or header streams within the specified range.
+		/// </remarks>
+		/// <returns>The messages.</returns>
+		/// <param name="startIndex">The index of the first stream to get.</param>
+		/// <param name="count">The number of streams to get.</param>
+		/// <param name="headersOnly"><c>true</c> if only the headers should be retrieved; otherwise, <c>false</c>.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task<IList<Stream>> GetStreamsAsync (int startIndex, int count, bool headersOnly = false, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Mark the specified message for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		void DeleteMessage (int index, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously mark the specified message for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task DeleteMessageAsync (int index, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Mark the specified messages for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		void DeleteMessages (IList<int> indexes, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously mark the specified messages for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task DeleteMessagesAsync (IList<int> indexes, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Mark the specified range of messages for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <param name="startIndex">The index of the first message to mark for deletion.</param>
+		/// <param name="count">The number of messages to mark for deletion.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		void DeleteMessages (int startIndex, int count, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously mark the specified range of messages for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="startIndex">The index of the first message to mark for deletion.</param>
+		/// <param name="count">The number of messages to mark for deletion.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task DeleteMessagesAsync (int startIndex, int count, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Mark all messages for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		void DeleteAllMessages (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously mark all messages for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task DeleteAllMessagesAsync (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Reset the state of all messages marked for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		void Reset (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously reset the state of all messages marked for deletion.
+		/// </summary>
+		/// <remarks>
+		/// Messages marked for deletion are not actually deleted until the session
+		/// is cleanly disconnected
+		/// (see <see cref="IMailService.Disconnect(bool, CancellationToken)"/>).
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task ResetAsync (CancellationToken cancellationToken = default (CancellationToken));
+	}
+}

src/MailKit/IMailStore.cs

@@ -0,0 +1,549 @@
+//
+// IMailStore.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+namespace MailKit {
+	/// <summary>
+	/// An interface for retreiving messages from a message store such as IMAP.
+	/// </summary>
+	/// <remarks>
+	/// Implemented by <see cref="MailKit.Net.Imap.ImapClient"/>.
+	/// </remarks>
+	public interface IMailStore : IMailService
+	{
+		/// <summary>
+		/// Get the personal namespaces.
+		/// </summary>
+		/// <remarks>
+		/// The personal folder namespaces contain a user's personal mailbox folders.
+		/// </remarks>
+		/// <value>The personal namespaces.</value>
+		FolderNamespaceCollection PersonalNamespaces { get; }
+
+		/// <summary>
+		/// Get the shared namespaces.
+		/// </summary>
+		/// <remarks>
+		/// The shared folder namespaces contain mailbox folders that are shared with the user.
+		/// </remarks>
+		/// <value>The shared namespaces.</value>
+		FolderNamespaceCollection SharedNamespaces { get; }
+
+		/// <summary>
+		/// Get the other namespaces.
+		/// </summary>
+		/// <remarks>
+		/// The other folder namespaces contain other mailbox folders.
+		/// </remarks>
+		/// <value>The other namespaces.</value>
+		FolderNamespaceCollection OtherNamespaces { get; }
+
+		/// <summary>
+		/// Get whether or not the mail store supports quotas.
+		/// </summary>
+		/// <remarks>
+		/// Gets whether or not the mail store supports quotas.
+		/// </remarks>
+		/// <value><c>true</c> if the mail store supports quotas; otherwise, <c>false</c>.</value>
+		bool SupportsQuotas { get; }
+
+		/// <summary>
+		/// Get the threading algorithms supported by the mail store.
+		/// </summary>
+		/// <remarks>
+		/// The threading algorithms are queried as part of the
+		/// <a href="Overload_MailKit_IMailStore_Connect.htm">Connect</a>
+		/// and <a href="Overload_MailKit_IMailStore_Authenticate.htm">Authenticate</a> methods.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The threading algorithms.</value>
+		HashSet<ThreadingAlgorithm> ThreadingAlgorithms { get; }
+
+		/// <summary>
+		/// Get the Inbox folder.
+		/// </summary>
+		/// <remarks>
+		/// The Inbox folder is the default folder and is typically the folder
+		/// where all new messages are delivered.
+		/// </remarks>
+		/// <value>The Inbox folder.</value>
+		IMailFolder Inbox { get; }
+
+		/// <summary>
+		/// Enable the quick resynchronization feature.
+		/// </summary>
+		/// <remarks>
+		/// <para>Enables quick resynchronization when a folder is opened using the
+		/// <see cref="IMailFolder.Open(FolderAccess,uint,ulong,System.Collections.Generic.IList&lt;UniqueId&gt;,System.Threading.CancellationToken)"/>
+		/// method.</para>
+		/// <para>If this feature is enabled, the <see cref="IMailFolder.MessageExpunged"/> event
+		/// is replaced with the <see cref="IMailFolder.MessagesVanished"/> event.</para>
+		/// <para>This method needs to be called immediately after
+		/// <see cref="IMailService.Authenticate(System.Net.ICredentials,System.Threading.CancellationToken)"/>,
+		/// before the opening of any folders.</para>
+		/// </remarks>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailService"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailService"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Quick resynchronization needs to be enabled before selecting a folder.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The mail store does not support quick resynchronization.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		void EnableQuickResync (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously enable the quick resynchronization feature.
+		/// </summary>
+		/// <remarks>
+		/// <para>Enables quick resynchronization when a folder is opened using the
+		/// <see cref="IMailFolder.Open(FolderAccess,uint,ulong,System.Collections.Generic.IList&lt;UniqueId&gt;,System.Threading.CancellationToken)"/>
+		/// method.</para>
+		/// <para>If this feature is enabled, the <see cref="IMailFolder.MessageExpunged"/> event
+		/// is replaced with the <see cref="IMailFolder.MessagesVanished"/> event.</para>
+		/// <para>This method needs to be called immediately after
+		/// <see cref="IMailService.Authenticate(System.Net.ICredentials,System.Threading.CancellationToken)"/>,
+		/// before the opening of any folders.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailService"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailService"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Quick resynchronization needs to be enabled before selecting a folder.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The mail store does not support quick resynchronization.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		Task EnableQuickResyncAsync (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the specified special folder.
+		/// </summary>
+		/// <remarks>
+		/// Not all message stores support the concept of special folders,
+		/// so this method may return <c>null</c>.
+		/// </remarks>
+		/// <returns>The folder if available; otherwise <c>null</c>.</returns>
+		/// <param name="folder">The type of special folder.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="folder"/> is out of range.
+		/// </exception>
+		IMailFolder GetFolder (SpecialFolder folder);
+
+		/// <summary>
+		/// Get the folder for the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// The main reason to get the toplevel folder in a namespace is
+		/// to list its child folders.
+		/// </remarks>
+		/// <returns>The folder.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The folder could not be found.
+		/// </exception>
+		IMailFolder GetFolder (FolderNamespace @namespace);
+
+		/// <summary>
+		/// Get all of the folders within the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Gets all of the folders within the specified namespace.
+		/// </remarks>
+		/// <returns>The folders.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <param name="subscribedOnly">If set to <c>true</c>, only subscribed folders will be listed.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailService"/> has been disposed.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailService"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailService"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The namespace folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		IList<IMailFolder> GetFolders (FolderNamespace @namespace, bool subscribedOnly, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get all of the folders within the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets all of the folders within the specified namespace.
+		/// </remarks>
+		/// <returns>The folders.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <param name="subscribedOnly">If set to <c>true</c>, only subscribed folders will be listed.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailService"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailService"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The namespace folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		Task<IList<IMailFolder>> GetFoldersAsync (FolderNamespace @namespace, bool subscribedOnly, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get all of the folders within the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Gets all of the folders within the specified namespace.
+		/// </remarks>
+		/// <returns>The folders.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <param name="items">The status items to pre-populate.</param>
+		/// <param name="subscribedOnly">If set to <c>true</c>, only subscribed folders will be listed.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailService"/> has been disposed.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailService"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailService"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The namespace folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		IList<IMailFolder> GetFolders (FolderNamespace @namespace, StatusItems items = StatusItems.None, bool subscribedOnly = false, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get all of the folders within the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets all of the folders within the specified namespace.
+		/// </remarks>
+		/// <returns>The folders.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <param name="items">The status items to pre-populate.</param>
+		/// <param name="subscribedOnly">If set to <c>true</c>, only subscribed folders will be listed.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailService"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailService"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The namespace folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		Task<IList<IMailFolder>> GetFoldersAsync (FolderNamespace @namespace, StatusItems items = StatusItems.None, bool subscribedOnly = false, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the folder for the specified path.
+		/// </summary>
+		/// <remarks>
+		/// Gets the folder for the specified path.
+		/// </remarks>
+		/// <returns>The folder.</returns>
+		/// <param name="path">The folder path.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="path"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		IMailFolder GetFolder (string path, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get the folder for the specified path.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the folder for the specified path.
+		/// </remarks>
+		/// <returns>The folder.</returns>
+		/// <param name="path">The folder path.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="path"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		Task<IMailFolder> GetFolderAsync (string path, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata value.</returns>
+		/// <param name="tag">The metadata tag.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		string GetMetadata (MetadataTag tag, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata value.</returns>
+		/// <param name="tag">The metadata tag.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<string> GetMetadataAsync (MetadataTag tag, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata.</returns>
+		/// <param name="tags">The metadata tags.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		MetadataCollection GetMetadata (IEnumerable<MetadataTag> tags, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata.</returns>
+		/// <param name="tags">The metadata tags.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<MetadataCollection> GetMetadataAsync (IEnumerable<MetadataTag> tags, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata.</returns>
+		/// <param name="options">The metadata options.</param>
+		/// <param name="tags">The metadata tags.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		MetadataCollection GetMetadata (MetadataOptions options, IEnumerable<MetadataTag> tags, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata.</returns>
+		/// <param name="options">The metadata options.</param>
+		/// <param name="tags">The metadata tags.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task<MetadataCollection> GetMetadataAsync (MetadataOptions options, IEnumerable<MetadataTag> tags, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Sets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Sets the specified metadata.
+		/// </remarks>
+		/// <param name="metadata">The metadata.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		void SetMetadata (MetadataCollection metadata, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously sets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously sets the specified metadata.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="metadata">The metadata.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		Task SetMetadataAsync (MetadataCollection metadata, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Occurs when a remote message store receives an alert message from the server.
+		/// </summary>
+		/// <remarks>
+		/// Some implementations, such as <see cref="MailKit.Net.Imap.ImapClient"/>,
+		/// will emit Alert events when they receive alert messages from the server.
+		/// </remarks>
+		event EventHandler<AlertEventArgs> Alert;
+
+		/// <summary>
+		/// Occurs when a folder is created.
+		/// </summary>
+		/// <remarks>
+		/// The <see cref="FolderCreated"/> event is emitted when a new folder is created.
+		/// </remarks>
+		event EventHandler<FolderCreatedEventArgs> FolderCreated;
+
+		/// <summary>
+		/// Occurs when metadata changes.
+		/// </summary>
+		/// <remarks>
+		/// The <see cref="MetadataChanged"/> event is emitted when metadata changes.
+		/// </remarks>
+		event EventHandler<MetadataChangedEventArgs> MetadataChanged;
+	}
+}

src/MailKit/IMailTransport.cs

@@ -0,0 +1,179 @@
+//
+// IMailTransport.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+using MimeKit;
+
+namespace MailKit {
+	/// <summary>
+	/// An interface for sending messages.
+	/// </summary>
+	/// <remarks>
+	/// An interface for sending messages.
+	/// </remarks>
+	public interface IMailTransport : IMailService
+	{
+		/// <summary>
+		/// Send the specified message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Sends the specified message.</para>
+		/// <para>The sender address is determined by checking the following
+		/// message headers (in order of precedence): Resent-Sender,
+		/// Resent-From, Sender, and From.</para>
+		/// <para>If either the Resent-Sender or Resent-From addresses are present,
+		/// the recipients are collected from the Resent-To, Resent-Cc, and
+		/// Resent-Bcc headers, otherwise the To, Cc, and Bcc headers are used.</para>
+		/// </remarks>
+		/// <param name="message">The message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		void Send (MimeMessage message, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously send the specified message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Asynchronously sends the specified message.</para>
+		/// <para>The sender address is determined by checking the following
+		/// message headers (in order of precedence): Resent-Sender,
+		/// Resent-From, Sender, and From.</para>
+		/// <para>If either the Resent-Sender or Resent-From addresses are present,
+		/// the recipients are collected from the Resent-To, Resent-Cc, and
+		/// Resent-Bcc headers, otherwise the To, Cc, and Bcc headers are used.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="message">The message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task SendAsync (MimeMessage message, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Send the specified message using the supplied sender and recipients.
+		/// </summary>
+		/// <remarks>
+		/// Sends the specified message using the supplied sender and recipients.
+		/// </remarks>
+		/// <param name="message">The message.</param>
+		/// <param name="sender">The mailbox address to use for sending the message.</param>
+		/// <param name="recipients">The mailbox addresses that should receive the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		void Send (MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously send the specified message using the supplied sender and recipients.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously sends the specified message using the supplied sender and recipients.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="message">The message.</param>
+		/// <param name="sender">The mailbox address to use for sending the message.</param>
+		/// <param name="recipients">The mailbox addresses that should receive the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task SendAsync (MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Send the specified message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Sends the specified message.</para>
+		/// <para>The sender address is determined by checking the following
+		/// message headers (in order of precedence): Resent-Sender,
+		/// Resent-From, Sender, and From.</para>
+		/// <para>If either the Resent-Sender or Resent-From addresses are present,
+		/// the recipients are collected from the Resent-To, Resent-Cc, and
+		/// Resent-Bcc headers, otherwise the To, Cc, and Bcc headers are used.</para>
+		/// </remarks>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="message">The message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		void Send (FormatOptions options, MimeMessage message, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously send the specified message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Asynchronously sends the specified message.</para>
+		/// <para>The sender address is determined by checking the following
+		/// message headers (in order of precedence): Resent-Sender,
+		/// Resent-From, Sender, and From.</para>
+		/// <para>If either the Resent-Sender or Resent-From addresses are present,
+		/// the recipients are collected from the Resent-To, Resent-Cc, and
+		/// Resent-Bcc headers, otherwise the To, Cc, and Bcc headers are used.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="message">The message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task SendAsync (FormatOptions options, MimeMessage message, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Send the specified message using the supplied sender and recipients.
+		/// </summary>
+		/// <remarks>
+		/// Sends the specified message using the supplied sender and recipients.
+		/// </remarks>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="message">The message.</param>
+		/// <param name="sender">The mailbox address to use for sending the message.</param>
+		/// <param name="recipients">The mailbox addresses that should receive the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		void Send (FormatOptions options, MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously send the specified message using the supplied sender and recipients.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously sends the specified message using the supplied sender and recipients.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="message">The message.</param>
+		/// <param name="sender">The mailbox address to use for sending the message.</param>
+		/// <param name="recipients">The mailbox addresses that should receive the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		Task SendAsync (FormatOptions options, MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Occurs when a message is successfully sent via the transport.
+		/// </summary>
+		/// <remarks>
+		/// The <see cref="MessageSent"/> event will be emitted each time a message is successfully sent.
+		/// </remarks>
+		event EventHandler<MessageSentEventArgs> MessageSent;
+	}
+}

src/MailKit/IMessageSummary.cs

@@ -0,0 +1,460 @@
+//
+// IMessageSummary.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Collections.Generic;
+
+using MimeKit;
+
+namespace MailKit {
+	/// <summary>
+	/// A summary of a message.
+	/// </summary>
+	/// <remarks>
+	/// <para>The <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a> and
+	/// <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a> methods
+	/// return lists of <see cref="IMessageSummary"/> items.</para>
+	/// <para>The properties of the <see cref="IMessageSummary"/> that will be available
+	/// depend on the <see cref="MessageSummaryItems"/> passed to the aformentioned method.</para>
+	/// </remarks>
+	public interface IMessageSummary
+	{
+		/// <summary>
+		/// Get the folder that the message belongs to.
+		/// </summary>
+		/// <remarks>
+		/// Gets the folder that the message belongs to, if available.
+		/// </remarks>
+		/// <value>The folder.</value>
+		IMailFolder Folder {
+			get;
+		}
+
+		/// <summary>
+		/// Get a bitmask of fields that have been populated.
+		/// </summary>
+		/// <remarks>
+		/// Gets a bitmask of fields that have been populated.
+		/// </remarks>
+		/// <value>The fields that have been populated.</value>
+		MessageSummaryItems Fields { get; }
+
+		/// <summary>
+		/// Gets the body structure of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>The body will be one of <see cref="BodyPartText"/>,
+		/// <see cref="BodyPartMessage"/>, <see cref="BodyPartBasic"/>,
+		/// or <see cref="BodyPartMultipart"/>.</para>
+		/// <para>This property will only be set if either the
+		/// <see cref="MessageSummaryItems.Body"/> flag or the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The body structure of the message.</value>
+		BodyPart Body { get; }
+
+		/// <summary>
+		/// Gets the text body part of the message if it exists.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the <c>text/plain</c> body part of the message.</para>
+		/// <para>This property will only be usable if the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadBodyParts"/>
+		/// </example>
+		/// <value>The text body if it exists; otherwise, <c>null</c>.</value>
+		BodyPartText TextBody { get; }
+
+		/// <summary>
+		/// Gets the html body part of the message if it exists.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the <c>text/html</c> body part of the message.</para>
+		/// <para>This property will only be usable if the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The html body if it exists; otherwise, <c>null</c>.</value>
+		BodyPartText HtmlBody { get; }
+
+		/// <summary>
+		/// Gets the body parts of the message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Traverses over the <see cref="Body"/>, enumerating all of the
+		/// <see cref="BodyPartBasic"/> objects.</para>
+		/// <para>This property will only be usable if either the
+		/// <see cref="MessageSummaryItems.Body"/> flag or the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The body parts.</value>
+		IEnumerable<BodyPartBasic> BodyParts { get; }
+
+		/// <summary>
+		/// Gets the attachments.
+		/// </summary>
+		/// <remarks>
+		/// <para>Traverses over the <see cref="Body"/>, enumerating all of the
+		/// <see cref="BodyPartBasic"/> objects that have a <c>Content-Disposition</c>
+		/// header set to <c>"attachment"</c>.</para>
+		/// <para>This property will only be usable if the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadBodyParts"/>
+		/// </example>
+		/// <value>The attachments.</value>
+		IEnumerable<BodyPartBasic> Attachments { get; }
+
+		/// <summary>
+		/// Gets the preview text of the message.
+		/// </summary>
+		/// <remarks>
+		/// <para>The preview text is a short snippet of the beginning of the message
+		/// text, typically shown in a mail client's message list to provide the user
+		/// with a sense of what the message is about.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.PreviewText"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The preview text.</value>
+		string PreviewText { get; }
+
+		/// <summary>
+		/// Gets the envelope of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>The envelope of a message contains information such as the
+		/// date the message was sent, the subject of the message,
+		/// the sender of the message, who the message was sent to,
+		/// which message(s) the message may be in reply to,
+		/// and the message id.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Envelope"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The envelope of the message.</value>
+		Envelope Envelope { get; }
+
+		/// <summary>
+		/// Gets the normalized subject.
+		/// </summary>
+		/// <remarks>
+		/// <para>A normalized <c>Subject</c> header value where prefixes such as <c>"Re:"</c>, <c>"Re[#]:"</c> and <c>"FWD:"</c> have been pruned.</para>
+		/// <para>This property is typically used for threading messages by subject.</para>
+		/// </remarks>
+		/// <value>The normalized subject.</value>
+		string NormalizedSubject { get; }
+
+		/// <summary>
+		/// Gets the Date header value.
+		/// </summary>
+		/// <remarks>
+		/// Gets the Date header value. If the Date header is not present, the arrival date is used.
+		/// If neither are known, <see cref="System.DateTimeOffset.MinValue"/> is returned.
+		/// </remarks>
+		/// <value>The date.</value>
+		DateTimeOffset Date { get; }
+
+		/// <summary>
+		/// Gets whether or not the message is a reply.
+		/// </summary>
+		/// <remarks>
+		/// This value should be based on whether the message subject contained any <c>"Re:"</c>, <c>"Re[#]:"</c> or <c>"FWD:"</c> prefixes.
+		/// </remarks>
+		/// <value><c>true</c> if the message is a reply; otherwise, <c>false</c>.</value>
+		bool IsReply { get; }
+
+		/// <summary>
+		/// Gets the message flags, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the message flags, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Flags"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The message flags.</value>
+		MessageFlags? Flags { get; }
+
+		/// <summary>
+		/// Gets the user-defined message flags, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the user-defined message flags, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Flags"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The user-defined message flags.</value>
+		HashSet<string> Keywords { get; }
+
+		/// <summary>
+		/// Gets the user-defined message flags, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the user-defined message flags, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Flags"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The user-defined message flags.</value>
+		[Obsolete ("Use Keywords instead.")]
+		HashSet<string> UserFlags { get; }
+
+		/// <summary>
+		/// Gets the message annotations, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the message annotations, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Annotations"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The message annotations.</value>
+		IList<Annotation> Annotations { get; }
+
+		/// <summary>
+		/// Gets the list of headers, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the list of headers, if available.</para>
+		/// <para>This property will only be set if <see cref="MessageSummaryItems.Headers"/>
+		/// is specified in a call to one of the
+		/// <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods or specific headers are requested via a one of the Fetch or FetchAsync methods
+		/// that accept list of specific headers to request for each message such as
+		/// <see cref="IMailFolder.Fetch(System.Collections.Generic.IList&lt;UniqueId&gt;,MessageSummaryItems,System.Collections.Generic.IEnumerable&lt;MimeKit.HeaderId&gt;,System.Threading.CancellationToken)"/>.
+		/// </para>
+		/// </remarks>
+		/// <value>The list of headers.</value>
+		HeaderList Headers { get; }
+
+		/// <summary>
+		/// Gets the internal date of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the internal date of the message (often the same date as found in the <c>Received</c> header), if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.InternalDate"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The internal date of the message.</value>
+		DateTimeOffset? InternalDate { get; }
+
+		/// <summary>
+		/// Gets the size of the message, in bytes, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the size of the message, in bytes, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Size"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The size of the message.</value>
+		uint? Size { get; }
+
+		/// <summary>
+		/// Gets the mod-sequence value for the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the mod-sequence value for the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.ModSeq"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The mod-sequence value.</value>
+		ulong? ModSeq { get; }
+
+		/// <summary>
+		/// Gets the message-ids that the message references, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the message-ids that the message references, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.References"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The references.</value>
+		MessageIdList References { get; }
+
+		/// <summary>
+		/// Get the globally unique identifier for the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the globally unique identifier of the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.EmailId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// <note type="info">This property maps to the <c>EMAILID</c> value defined in the
+		/// <a href="https://tools.ietf.org/html/rfc8474">OBJECTID</a> extension.</note>
+		/// </remarks>
+		/// <value>The globally unique message identifier.</value>
+		string EmailId { get; }
+
+		/// <summary>
+		/// Get the globally unique identifier for the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the globally unique identifier of the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.EmailId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// <note type="info">This property maps to the <c>EMAILID</c> value defined in the
+		/// <a href="https://tools.ietf.org/html/rfc8474">OBJECTID</a> extension.</note>
+		/// </remarks>
+		/// <value>The globally unique message identifier.</value>
+		[Obsolete ("Use EmailId instead.")]
+		string Id { get; }
+
+		/// <summary>
+		/// Get the globally unique thread identifier for the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the globally unique thread identifier for the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.ThreadId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// <note type="info">This property maps to the <c>THREADID</c> value defined in the
+		/// <a href="https://tools.ietf.org/html/rfc8474">OBJECTID</a> extension.</note>
+		/// </remarks>
+		/// <value>The globally unique thread identifier.</value>
+		string ThreadId { get; }
+
+		/// <summary>
+		/// Gets the unique identifier of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the unique identifier of the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.UniqueId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The uid of the message.</value>
+		UniqueId UniqueId { get; }
+
+		/// <summary>
+		/// Gets the index of the message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the index of the message.</para>
+		/// <para>This property is always set.</para>
+		/// </remarks>
+		/// <value>The index of the message.</value>
+		int Index { get; }
+
+		#region GMail extension properties
+
+		/// <summary>
+		/// Gets the GMail message identifier, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the GMail message identifier, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.GMailMessageId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The GMail message identifier.</value>
+		ulong? GMailMessageId { get; }
+
+		/// <summary>
+		/// Gets the GMail thread identifier, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the GMail thread identifier, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.GMailThreadId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The GMail thread identifier.</value>
+		ulong? GMailThreadId { get; }
+
+		/// <summary>
+		/// Gets the list of GMail labels, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the list of GMail labels, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.GMailLabels"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The GMail labels.</value>
+		IList<string> GMailLabels { get; }
+
+		#endregion
+	}
+}

src/MailKit/IProtocolLogger.cs

@@ -0,0 +1,116 @@
+//
+// IProtocolLogger.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// An interface for logging the communication between a client and server.
+	/// </summary>
+	/// <remarks>
+	/// An interface for logging the communication between a client and server.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\SmtpExamples.cs" region="ProtocolLogger" />
+	/// </example>
+	public interface IProtocolLogger : IDisposable
+	{
+		/// <summary>
+		/// Logs a connection to the specified URI.
+		/// </summary>
+		/// <remarks>
+		/// Logs a connection to the specified URI.
+		/// </remarks>
+		/// <param name="uri">The URI.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="uri"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The logger has been disposed.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		void LogConnect (Uri uri);
+
+		/// <summary>
+		/// Logs a sequence of bytes sent by the client.
+		/// </summary>
+		/// <remarks>
+		/// <para>Logs a sequence of bytes sent by the client.</para>
+		/// <para><see cref="LogClient(byte[], int, int)"/> is called by the <see cref="IMailService"/> upon every successful
+		/// write operation to its underlying network stream, passing the exact same <paramref name="buffer"/>,
+		/// <paramref name="offset"/>, and <paramref name="count"/> arguments to the logging function.</para>
+		/// </remarks>
+		/// <param name='buffer'>The buffer to log.</param>
+		/// <param name='offset'>The offset of the first byte to log.</param>
+		/// <param name='count'>The number of bytes to log.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The logger has been disposed.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		void LogClient (byte[] buffer, int offset, int count);
+
+		/// <summary>
+		/// Logs a sequence of bytes sent by the server.
+		/// </summary>
+		/// <remarks>
+		/// <para>Logs a sequence of bytes sent by the server.</para>
+		/// <para><see cref="LogServer(byte[], int, int)"/> is called by the <see cref="IMailService"/> upon every successful
+		/// read of its underlying network stream with the exact buffer that was read.</para>
+		/// </remarks>
+		/// <param name='buffer'>The buffer to log.</param>
+		/// <param name='offset'>The offset of the first byte to log.</param>
+		/// <param name='count'>The number of bytes to log.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="buffer"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="offset"/> is less than zero or greater than the length of <paramref name="buffer"/>.</para>
+		/// <para>-or-</para>
+		/// <para>The <paramref name="buffer"/> is not large enough to contain <paramref name="count"/> bytes strting
+		/// at the specified <paramref name="offset"/>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The logger has been disposed.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		void LogServer (byte[] buffer, int offset, int count);
+	}
+}

src/MailKit/ITransferProgress.cs

@@ -0,0 +1,58 @@
+//
+// ITransferProgress.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit {
+	/// <summary>
+	/// An interface for reporting progress of uploading or downloading messages.
+	/// </summary>
+	/// <remarks>
+	/// An interface for reporting progress of uploading or downloading messages.
+	/// </remarks>
+	public interface ITransferProgress
+	{
+		/// <summary>
+		/// Report the progress of the transfer operation.
+		/// </summary>
+		/// <remarks>
+		/// <para>Reports the progress of the transfer operation.</para>
+		/// <para>This method is only used if the operation knows the size
+		/// of the message, part, or stream being transferred without doing
+		/// extra work to calculate it.</para>
+		/// </remarks>
+		/// <param name="bytesTransferred">The number of bytes transferred.</param>
+		/// <param name="totalSize">The total size, in bytes, of the message, part, or stream being transferred.</param>
+		void Report (long bytesTransferred, long totalSize);
+
+		/// <summary>
+		/// Report the progress of the transfer operation.
+		/// </summary>
+		/// <remarks>
+		/// <para>Reports the progress of the transfer operation.</para>
+		/// </remarks>
+		/// <param name="bytesTransferred">The number of bytes transferred.</param>
+		void Report (long bytesTransferred);
+	}
+}

src/MailKit/MailStore.cs

@@ -0,0 +1,872 @@
+//
+// MailStore.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+namespace MailKit {
+	/// <summary>
+	/// An abstract mail store implementation.
+	/// </summary>
+	/// <remarks>
+	/// An abstract mail store implementation.
+	/// </remarks>
+	public abstract class MailStore : MailService, IMailStore
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MailStore"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Initializes a new instance of the <see cref="MailKit.MailStore"/> class.
+		/// </remarks>
+		/// <param name="protocolLogger">The protocol logger.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="protocolLogger"/> is <c>null</c>.
+		/// </exception>
+		protected MailStore (IProtocolLogger protocolLogger) : base (protocolLogger)
+		{
+		}
+
+		/// <summary>
+		/// Gets the personal namespaces.
+		/// </summary>
+		/// <remarks>
+		/// The personal folder namespaces contain a user's personal mailbox folders.
+		/// </remarks>
+		/// <value>The personal namespaces.</value>
+		public abstract FolderNamespaceCollection PersonalNamespaces {
+			get;
+		}
+
+		/// <summary>
+		/// Gets the shared namespaces.
+		/// </summary>
+		/// <remarks>
+		/// The shared folder namespaces contain mailbox folders that are shared with the user.
+		/// </remarks>
+		/// <value>The shared namespaces.</value>
+		public abstract FolderNamespaceCollection SharedNamespaces {
+			get;
+		}
+
+		/// <summary>
+		/// Gets the other namespaces.
+		/// </summary>
+		/// <remarks>
+		/// The other folder namespaces contain other mailbox folders.
+		/// </remarks>
+		/// <value>The other namespaces.</value>
+		public abstract FolderNamespaceCollection OtherNamespaces {
+			get;
+		}
+
+		/// <summary>
+		/// Get whether or not the mail store supports quotas.
+		/// </summary>
+		/// <remarks>
+		/// Gets whether or not the mail store supports quotas.
+		/// </remarks>
+		/// <value><c>true</c> if the mail store supports quotas; otherwise, <c>false</c>.</value>
+		public abstract bool SupportsQuotas {
+			get;
+		}
+
+		/// <summary>
+		/// Get the threading algorithms supported by the mail store.
+		/// </summary>
+		/// <remarks>
+		/// The threading algorithms are queried as part of the
+		/// <a href="Overload_MailKit_MailStore_Connect.htm">Connect</a>
+		/// and <a href="Overload_MailKit_MailStore_Authenticate.htm">Authenticate</a> methods.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The supported threading algorithms.</value>
+		public abstract HashSet<ThreadingAlgorithm> ThreadingAlgorithms {
+			get;
+		}
+
+		/// <summary>
+		/// Get the Inbox folder.
+		/// </summary>
+		/// <remarks>
+		/// <para>The Inbox folder is the default folder and always exists on the mail store.</para>
+		/// <note type="note">This property will only be available after the client has been authenticated.</note>
+		/// </remarks>
+		/// <value>The Inbox folder.</value>
+		public abstract IMailFolder Inbox {
+			get;
+		}
+
+		/// <summary>
+		/// Enable the quick resynchronization feature.
+		/// </summary>
+		/// <remarks>
+		/// <para>Enables quick resynchronization when a folder is opened using the
+		/// <see cref="MailFolder.Open(FolderAccess,uint,ulong,System.Collections.Generic.IList&lt;UniqueId&gt;,System.Threading.CancellationToken)"/>
+		/// method.</para>
+		/// <para>If this feature is enabled, the <see cref="MailFolder.MessageExpunged"/> event is replaced
+		/// with the <see cref="MailFolder.MessagesVanished"/> event.</para>
+		/// <para>This method needs to be called immediately after calling one of the
+		/// <a href="Overload_MailKit_MailService_Authenticate.htm">Authenticate</a> methods, before
+		/// opening any folders.</para>
+		/// </remarks>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Quick resynchronization needs to be enabled before selecting a folder.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The mail store does not support quick resynchronization.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract void EnableQuickResync (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously enable the quick resynchronization feature.
+		/// </summary>
+		/// <remarks>
+		/// <para>Enables quick resynchronization when a folder is opened using the
+		/// <see cref="MailFolder.Open(FolderAccess,uint,ulong,System.Collections.Generic.IList&lt;UniqueId&gt;,System.Threading.CancellationToken)"/>
+		/// method.</para>
+		/// <para>If this feature is enabled, the <see cref="MailFolder.MessageExpunged"/> event is replaced
+		/// with the <see cref="MailFolder.MessagesVanished"/> event.</para>
+		/// <para>This method needs to be called immediately after calling one of the
+		/// <a href="Overload_MailKit_MailService_Authenticate.htm">Authenticate</a> methods, before
+		/// opening any folders.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Quick resynchronization needs to be enabled before selecting a folder.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The mail store does not support quick resynchronization.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract Task EnableQuickResyncAsync (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the specified special folder.
+		/// </summary>
+		/// <remarks>
+		/// Not all mail stores support special folders. Each implementation
+		/// should provide a way to determine if special folders are supported.
+		/// </remarks>
+		/// <returns>The folder if available; otherwise <c>null</c>.</returns>
+		/// <param name="folder">The type of special folder.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="folder"/> is out of range.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		public abstract IMailFolder GetFolder (SpecialFolder folder);
+
+		/// <summary>
+		/// Get the folder for the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Gets the folder for the specified namespace.
+		/// </remarks>
+		/// <returns>The folder.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The folder could not be found.
+		/// </exception>
+		public abstract IMailFolder GetFolder (FolderNamespace @namespace);
+
+		/// <summary>
+		/// Get all of the folders within the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Gets all of the folders within the specified namespace.
+		/// </remarks>
+		/// <returns>The folders.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <param name="subscribedOnly">If set to <c>true</c>, only subscribed folders will be listed.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public virtual IList<IMailFolder> GetFolders (FolderNamespace @namespace, bool subscribedOnly, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return GetFolders (@namespace, StatusItems.None, subscribedOnly, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously get all of the folders within the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets all of the folders within the specified namespace.
+		/// </remarks>
+		/// <returns>The folders.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <param name="subscribedOnly">If set to <c>true</c>, only subscribed folders will be listed.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public virtual Task<IList<IMailFolder>> GetFoldersAsync (FolderNamespace @namespace, bool subscribedOnly, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return GetFoldersAsync (@namespace, StatusItems.None, subscribedOnly, cancellationToken);
+		}
+
+		/// <summary>
+		/// Get all of the folders within the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Gets all of the folders within the specified namespace.
+		/// </remarks>
+		/// <returns>The folders.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <param name="items">The status items to pre-populate.</param>
+		/// <param name="subscribedOnly">If set to <c>true</c>, only subscribed folders will be listed.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract IList<IMailFolder> GetFolders (FolderNamespace @namespace, StatusItems items = StatusItems.None, bool subscribedOnly = false, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get all of the folders within the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets all of the folders within the specified namespace.
+		/// </remarks>
+		/// <returns>The folders.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <param name="items">The status items to pre-populate.</param>
+		/// <param name="subscribedOnly">If set to <c>true</c>, only subscribed folders will be listed.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract Task<IList<IMailFolder>> GetFoldersAsync (FolderNamespace @namespace, StatusItems items = StatusItems.None, bool subscribedOnly = false, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the folder for the specified path.
+		/// </summary>
+		/// <remarks>
+		/// Gets the folder for the specified path.
+		/// </remarks>
+		/// <returns>The folder.</returns>
+		/// <param name="path">The folder path.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="path"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract IMailFolder GetFolder (string path, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get the folder for the specified path.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the folder for the specified path.
+		/// </remarks>
+		/// <returns>The folder.</returns>
+		/// <param name="path">The folder path.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="path"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="MailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract Task<IMailFolder> GetFolderAsync (string path, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata value.</returns>
+		/// <param name="tag">The metadata tag.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The folder does not support metadata.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract string GetMetadata (MetadataTag tag, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata value.</returns>
+		/// <param name="tag">The metadata tag.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The folder does not support metadata.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract Task<string> GetMetadataAsync (MetadataTag tag, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata.</returns>
+		/// <param name="tags">The metadata tags.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="tags"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The folder does not support metadata.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public virtual MetadataCollection GetMetadata (IEnumerable<MetadataTag> tags, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return GetMetadata (new MetadataOptions (), tags, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata.</returns>
+		/// <param name="tags">The metadata tags.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="tags"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The folder does not support metadata.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public virtual Task<MetadataCollection> GetMetadataAsync (IEnumerable<MetadataTag> tags, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return GetMetadataAsync (new MetadataOptions (), tags, cancellationToken);
+		}
+
+		/// <summary>
+		/// Gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata.</returns>
+		/// <param name="options">The metadata options.</param>
+		/// <param name="tags">The metadata tags.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="options"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="tags"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The folder does not support metadata.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract MetadataCollection GetMetadata (MetadataOptions options, IEnumerable<MetadataTag> tags, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata.</returns>
+		/// <param name="options">The metadata options.</param>
+		/// <param name="tags">The metadata tags.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="options"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="tags"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The folder does not support metadata.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract Task<MetadataCollection> GetMetadataAsync (MetadataOptions options, IEnumerable<MetadataTag> tags, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Sets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Sets the specified metadata.
+		/// </remarks>
+		/// <param name="metadata">The metadata.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="metadata"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The folder does not support metadata.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract void SetMetadata (MetadataCollection metadata, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously sets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously sets the specified metadata.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="metadata">The metadata.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="metadata"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="IMailStore"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="IMailStore"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="IMailStore"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The folder does not support metadata.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The command failed.
+		/// </exception>
+		public abstract Task SetMetadataAsync (MetadataCollection metadata, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Occurs when a remote message store receives an alert message from the server.
+		/// </summary>
+		/// <remarks>
+		/// The <see cref="Alert"/> event is raised whenever the mail server sends an
+		/// alert message.
+		/// </remarks>
+		public event EventHandler<AlertEventArgs> Alert;
+
+		/// <summary>
+		/// Raise the alert event.
+		/// </summary>
+		/// <remarks>
+		/// Raises the alert event.
+		/// </remarks>
+		/// <param name="message">The alert message.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="message"/> is <c>null</c>.
+		/// </exception>
+		protected virtual void OnAlert (string message)
+		{
+			var handler = Alert;
+
+			if (handler != null)
+				handler (this, new AlertEventArgs (message));
+		}
+
+		/// <summary>
+		/// Occurs when a folder is created.
+		/// </summary>
+		/// <remarks>
+		/// The <see cref="FolderCreated"/> event is emitted when a new folder is created.
+		/// </remarks>
+		public event EventHandler<FolderCreatedEventArgs> FolderCreated;
+
+		/// <summary>
+		/// Raise the folder created event.
+		/// </summary>
+		/// <remarks>
+		/// Raises the folder created event.
+		/// </remarks>
+		/// <param name="folder">The folder that was just created.</param>
+		protected virtual void OnFolderCreated (IMailFolder folder)
+		{
+			var handler = FolderCreated;
+
+			if (handler != null)
+				handler (this, new FolderCreatedEventArgs (folder));
+		}
+
+		/// <summary>
+		/// Occurs when metadata changes.
+		/// </summary>
+		/// <remarks>
+		/// The <see cref="MetadataChanged"/> event is emitted when metadata changes.
+		/// </remarks>
+		public event EventHandler<MetadataChangedEventArgs> MetadataChanged;
+
+		/// <summary>
+		/// Raise the metadata changed event.
+		/// </summary>
+		/// <remarks>
+		/// Raises the metadata changed event.
+		/// </remarks>
+		/// <param name="metadata">The metadata that changed.</param>
+		protected virtual void OnMetadataChanged (Metadata metadata)
+		{
+			var handler = MetadataChanged;
+
+			if (handler != null)
+				handler (this, new MetadataChangedEventArgs (metadata));
+		}
+	}
+}

src/MailKit/MailTransport.cs

@@ -0,0 +1,507 @@
+//
+// MailTransport.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+using MimeKit;
+
+namespace MailKit {
+	/// <summary>
+	/// An abstract mail transport implementation.
+	/// </summary>
+	/// <remarks>
+	/// An abstract mail transport implementation.
+	/// </remarks>
+	public abstract class MailTransport : MailService, IMailTransport
+	{
+		static readonly FormatOptions DefaultOptions;
+
+		static MailTransport ()
+		{
+			var options = FormatOptions.Default.Clone ();
+			options.HiddenHeaders.Add (HeaderId.ContentLength);
+			options.HiddenHeaders.Add (HeaderId.ResentBcc);
+			options.HiddenHeaders.Add (HeaderId.Bcc);
+			options.NewLineFormat = NewLineFormat.Dos;
+
+			DefaultOptions = options;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MailTransport"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Initializes a new instance of the <see cref="MailKit.MailTransport"/> class.
+		/// </remarks>
+		/// <param name="protocolLogger">The protocol logger.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="protocolLogger"/> is <c>null</c>.
+		/// </exception>
+		protected MailTransport (IProtocolLogger protocolLogger) : base (protocolLogger)
+		{
+		}
+
+		/// <summary>
+		/// Send the specified message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Sends the specified message.</para>
+		/// <para>The sender address is determined by checking the following
+		/// message headers (in order of precedence): Resent-Sender,
+		/// Resent-From, Sender, and From.</para>
+		/// <para>If either the Resent-Sender or Resent-From addresses are present,
+		/// the recipients are collected from the Resent-To, Resent-Cc, and
+		/// Resent-Bcc headers, otherwise the To, Cc, and Bcc headers are used.</para>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\SmtpExamples.cs" region="SendMessage"/>
+		/// </example>
+		/// <param name="message">The message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="message"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailTransport"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailTransport"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// Authentication is required before sending a message.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// <para>A sender has not been specified.</para>
+		/// <para>-or-</para>
+		/// <para>No recipients have been specified.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation has been canceled.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The send command failed.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol exception occurred.
+		/// </exception>
+		public virtual void Send (MimeMessage message, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null)
+		{
+			Send (DefaultOptions, message, cancellationToken, progress);
+		}
+
+		/// <summary>
+		/// Asynchronously send the specified message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Asynchronously sends the specified message.</para>
+		/// <para>The sender address is determined by checking the following
+		/// message headers (in order of precedence): Resent-Sender,
+		/// Resent-From, Sender, and From.</para>
+		/// <para>If either the Resent-Sender or Resent-From addresses are present,
+		/// the recipients are collected from the Resent-To, Resent-Cc, and
+		/// Resent-Bcc headers, otherwise the To, Cc, and Bcc headers are used.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="message">The message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="message"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailTransport"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailTransport"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// Authentication is required before sending a message.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// <para>A sender has not been specified.</para>
+		/// <para>-or-</para>
+		/// <para>No recipients have been specified.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation has been canceled.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The send command failed.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol exception occurred.
+		/// </exception>
+		public virtual Task SendAsync (MimeMessage message, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null)
+		{
+			return SendAsync (DefaultOptions, message, cancellationToken, progress);
+		}
+
+		/// <summary>
+		/// Send the specified message using the supplied sender and recipients.
+		/// </summary>
+		/// <remarks>
+		/// Sends the specified message using the supplied sender and recipients.
+		/// </remarks>
+		/// <param name="message">The message.</param>
+		/// <param name="sender">The mailbox address to use for sending the message.</param>
+		/// <param name="recipients">The mailbox addresses that should receive the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="message"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="sender"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="recipients"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailTransport"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailTransport"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// Authentication is required before sending a message.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// <para>A sender has not been specified.</para>
+		/// <para>-or-</para>
+		/// <para>No recipients have been specified.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation has been canceled.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The send command failed.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol exception occurred.
+		/// </exception>
+		public virtual void Send (MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null)
+		{
+			Send (DefaultOptions, message, sender, recipients, cancellationToken, progress);
+		}
+
+		/// <summary>
+		/// Asynchronously send the specified message using the supplied sender and recipients.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously sends the specified message using the supplied sender and recipients.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="message">The message.</param>
+		/// <param name="sender">The mailbox address to use for sending the message.</param>
+		/// <param name="recipients">The mailbox addresses that should receive the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="message"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="sender"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="recipients"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailTransport"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailTransport"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// Authentication is required before sending a message.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// <para>A sender has not been specified.</para>
+		/// <para>-or-</para>
+		/// <para>No recipients have been specified.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation has been canceled.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The send command failed.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol exception occurred.
+		/// </exception>
+		public virtual Task SendAsync (MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null)
+		{
+			return SendAsync (DefaultOptions, message, sender, recipients, cancellationToken, progress);
+		}
+
+		/// <summary>
+		/// Send the specified message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Sends the specified message.</para>
+		/// <para>The sender address is determined by checking the following
+		/// message headers (in order of precedence): Resent-Sender,
+		/// Resent-From, Sender, and From.</para>
+		/// <para>If either the Resent-Sender or Resent-From addresses are present,
+		/// the recipients are collected from the Resent-To, Resent-Cc, and
+		/// Resent-Bcc headers, otherwise the To, Cc, and Bcc headers are used.</para>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\SmtpExamples.cs" region="SendMessageWithOptions"/>
+		/// </example>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="message">The message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="options"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="message"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailTransport"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailTransport"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// Authentication is required before sending a message.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// <para>A sender has not been specified.</para>
+		/// <para>-or-</para>
+		/// <para>No recipients have been specified.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation has been canceled.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <para>Internationalized formatting was requested but is not supported by the transport.</para>
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The send command failed.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol exception occurred.
+		/// </exception>
+		public abstract void Send (FormatOptions options, MimeMessage message, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously send the specified message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Asynchronously sends the specified message.</para>
+		/// <para>The sender address is determined by checking the following
+		/// message headers (in order of precedence): Resent-Sender,
+		/// Resent-From, Sender, and From.</para>
+		/// <para>If either the Resent-Sender or Resent-From addresses are present,
+		/// the recipients are collected from the Resent-To, Resent-Cc, and
+		/// Resent-Bcc headers, otherwise the To, Cc, and Bcc headers are used.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="message">The message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="options"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="message"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailTransport"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailTransport"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// Authentication is required before sending a message.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// <para>A sender has not been specified.</para>
+		/// <para>-or-</para>
+		/// <para>No recipients have been specified.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation has been canceled.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <para>Internationalized formatting was requested but is not supported by the transport.</para>
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The send command failed.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol exception occurred.
+		/// </exception>
+		public abstract Task SendAsync (FormatOptions options, MimeMessage message, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Send the specified message using the supplied sender and recipients.
+		/// </summary>
+		/// <remarks>
+		/// Sends the specified message using the supplied sender and recipients.
+		/// </remarks>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="message">The message.</param>
+		/// <param name="sender">The mailbox address to use for sending the message.</param>
+		/// <param name="recipients">The mailbox addresses that should receive the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="options"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="message"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="sender"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="recipients"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailTransport"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailTransport"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// Authentication is required before sending a message.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// <para>A sender has not been specified.</para>
+		/// <para>-or-</para>
+		/// <para>No recipients have been specified.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation has been canceled.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <para>Internationalized formatting was requested but is not supported by the transport.</para>
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The send command failed.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol exception occurred.
+		/// </exception>
+		public abstract void Send (FormatOptions options, MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously send the specified message using the supplied sender and recipients.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously sends the specified message using the supplied sender and recipients.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="message">The message.</param>
+		/// <param name="sender">The mailbox address to use for sending the message.</param>
+		/// <param name="recipients">The mailbox addresses that should receive the message.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="options"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="message"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="sender"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="recipients"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="MailTransport"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="MailTransport"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// Authentication is required before sending a message.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// <para>A sender has not been specified.</para>
+		/// <para>-or-</para>
+		/// <para>No recipients have been specified.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation has been canceled.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <para>Internationalized formatting was requested but is not supported by the transport.</para>
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="CommandException">
+		/// The send command failed.
+		/// </exception>
+		/// <exception cref="ProtocolException">
+		/// A protocol exception occurred.
+		/// </exception>
+		public abstract Task SendAsync (FormatOptions options, MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Occurs when a message is successfully sent via the transport.
+		/// </summary>
+		/// <remarks>
+		/// The <see cref="MessageSent"/> event will be emitted each time a message is successfully sent.
+		/// </remarks>
+		public event EventHandler<MessageSentEventArgs> MessageSent;
+
+		/// <summary>
+		/// Raise the message sent event.
+		/// </summary>
+		/// <remarks>
+		/// Raises the message sent event.
+		/// </remarks>
+		/// <param name="e">The message sent event args.</param>
+		protected virtual void OnMessageSent (MessageSentEventArgs e)
+		{
+			var handler = MessageSent;
+
+			if (handler != null)
+				handler (this, e);
+		}
+	}
+}

src/MailKit/MessageEventArgs.cs

@@ -0,0 +1,98 @@
+//
+// MessageEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args used when the state of a message changes.
+	/// </summary>
+	/// <remarks>
+	/// Event args used when the state of a message changes.
+	/// </remarks>
+	public class MessageEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.MessageEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageEventArgs (int index)
+		{
+			if (index < 0)
+				throw new ArgumentOutOfRangeException (nameof (index));
+
+			Index = index;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.MessageEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="uid">The unique id of the message.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageEventArgs (int index, UniqueId uid)
+		{
+			if (index < 0)
+				throw new ArgumentOutOfRangeException (nameof (index));
+
+			Index = index;
+			UniqueId = uid;
+		}
+
+		/// <summary>
+		/// Gets the index of the message that changed.
+		/// </summary>
+		/// <remarks>
+		/// Gets the index of the message that changed.
+		/// </remarks>
+		/// <value>The index of the message.</value>
+		public int Index {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the unique ID of the message that changed, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the unique ID of the message that changed, if available.
+		/// </remarks>
+		/// <value>The unique ID of the message.</value>
+		public UniqueId? UniqueId {
+			get; internal set;
+		}
+	}
+}

src/MailKit/MessageFlags.cs

@@ -0,0 +1,78 @@
+//
+// MessageFlags.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// An enumeration of message flags.
+	/// </summary>
+	/// <remarks>
+	/// An enumeration of message flags.
+	/// </remarks>
+	[Flags]
+	public enum MessageFlags {
+		/// <summary>
+		/// No message flags are set.
+		/// </summary>
+		None        = 0,
+
+		/// <summary>
+		/// The message has been read.
+		/// </summary>
+		Seen        = 1 << 0,
+
+		/// <summary>
+		/// The message has been answered (replied to).
+		/// </summary>
+		Answered    = 1 << 1,
+
+		/// <summary>
+		/// The message has been flagged for importance.
+		/// </summary>
+		Flagged     = 1 << 2,
+
+		/// <summary>
+		/// The message has been marked for deletion.
+		/// </summary>
+		Deleted     = 1 << 3,
+
+		/// <summary>
+		/// The message is marked as a draft.
+		/// </summary>
+		Draft       = 1 << 4,
+
+		/// <summary>
+		/// The message has just recently arrived in the folder.
+		/// </summary>
+		Recent      = 1 << 5,
+
+		/// <summary>
+		/// User-defined flags are allowed by the folder.
+		/// </summary>
+		UserDefined = 1 << 6,
+	}
+}

src/MailKit/MessageFlagsChangedEventArgs.cs

@@ -0,0 +1,269 @@
+//
+// MessageFlagsChangedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Collections.Generic;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args for the <see cref="IMailFolder.MessageFlagsChanged"/> event.
+	/// </summary>
+	/// <remarks>
+	/// Event args for the <see cref="IMailFolder.MessageFlagsChanged"/> event.
+	/// </remarks>
+	public class MessageFlagsChangedEventArgs : MessageEventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		internal MessageFlagsChangedEventArgs (int index) : base (index)
+		{
+			Keywords = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="flags">The message flags.</param>
+		public MessageFlagsChangedEventArgs (int index, MessageFlags flags) : base (index)
+		{
+			Keywords = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
+			Flags = flags;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="flags">The message flags.</param>
+		/// <param name="keywords">The user-defined message flags.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="keywords"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageFlagsChangedEventArgs (int index, MessageFlags flags, HashSet<string> keywords) : base (index)
+		{
+			if (keywords == null)
+				throw new ArgumentNullException (nameof (keywords));
+
+			Keywords = keywords;
+			Flags = flags;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="flags">The message flags.</param>
+		/// <param name="modseq">The modification sequence value.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageFlagsChangedEventArgs (int index, MessageFlags flags, ulong modseq) : base (index)
+		{
+			Keywords = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
+			ModSeq = modseq;
+			Flags = flags;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="flags">The message flags.</param>
+		/// <param name="keywords">The user-defined message flags.</param>
+		/// <param name="modseq">The modification sequence value.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="keywords"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageFlagsChangedEventArgs (int index, MessageFlags flags, HashSet<string> keywords, ulong modseq) : base (index)
+		{
+			if (keywords == null)
+				throw new ArgumentNullException (nameof (keywords));
+
+			Keywords = keywords;
+			ModSeq = modseq;
+			Flags = flags;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="uid">The unique id of the message.</param>
+		/// <param name="flags">The message flags.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageFlagsChangedEventArgs (int index, UniqueId uid, MessageFlags flags) : base (index, uid)
+		{
+			Keywords = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
+			Flags = flags;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="uid">The unique id of the message.</param>
+		/// <param name="flags">The message flags.</param>
+		/// <param name="keywords">The user-defined message flags.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="keywords"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageFlagsChangedEventArgs (int index, UniqueId uid, MessageFlags flags, HashSet<string> keywords) : base (index, uid)
+		{
+			if (keywords == null)
+				throw new ArgumentNullException (nameof (keywords));
+
+			Keywords = keywords;
+			Flags = flags;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="uid">The unique id of the message.</param>
+		/// <param name="flags">The message flags.</param>
+		/// <param name="modseq">The modification sequence value.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageFlagsChangedEventArgs (int index, UniqueId uid, MessageFlags flags, ulong modseq) : base (index, uid)
+		{
+			Keywords = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
+			ModSeq = modseq;
+			Flags = flags;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="uid">The unique id of the message.</param>
+		/// <param name="flags">The message flags.</param>
+		/// <param name="keywords">The user-defined message flags.</param>
+		/// <param name="modseq">The modification sequence value.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="keywords"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageFlagsChangedEventArgs (int index, UniqueId uid, MessageFlags flags, HashSet<string> keywords, ulong modseq) : base (index, uid)
+		{
+			if (keywords == null)
+				throw new ArgumentNullException (nameof (keywords));
+
+			Keywords = keywords;
+			ModSeq = modseq;
+			Flags = flags;
+		}
+
+		/// <summary>
+		/// Gets the updated message flags.
+		/// </summary>
+		/// <remarks>
+		/// Gets the updated message flags.
+		/// </remarks>
+		/// <value>The updated message flags.</value>
+		public MessageFlags Flags {
+			get; internal set;
+		}
+
+		/// <summary>
+		/// Gets the updated user-defined message flags.
+		/// </summary>
+		/// <remarks>
+		/// Gets the updated user-defined message flags.
+		/// </remarks>
+		/// <value>The updated user-defined message flags.</value>
+		public HashSet<string> Keywords {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the updated user-defined message flags.
+		/// </summary>
+		/// <remarks>
+		/// Gets the updated user-defined message flags.
+		/// </remarks>
+		/// <value>The updated user-defined message flags.</value>
+		[Obsolete ("Use Keywords instead.")]
+		public HashSet<string> UserFlags {
+			get { return Keywords; }
+		}
+
+		/// <summary>
+		/// Gets the updated mod-sequence value of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the updated mod-sequence value of the message, if available.
+		/// </remarks>
+		/// <value>The mod-sequence value.</value>
+		public ulong? ModSeq {
+			get; internal set;
+		}
+	}
+}

src/MailKit/MessageLabelsChangedEventArgs.cs

@@ -0,0 +1,170 @@
+//
+// LabelsChangedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args for the <see cref="IMailFolder.MessageLabelsChanged"/> event.
+	/// </summary>
+	/// <remarks>
+	/// Event args for the <see cref="IMailFolder.MessageLabelsChanged"/> event.
+	/// </remarks>
+	public class MessageLabelsChangedEventArgs : MessageEventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageLabelsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageLabelsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		internal MessageLabelsChangedEventArgs (int index) : base (index)
+		{
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageLabelsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageLabelsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="labels">The message labels.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="labels"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageLabelsChangedEventArgs (int index, IList<string> labels) : base (index)
+		{
+			if (labels == null)
+				throw new ArgumentNullException (nameof (labels));
+
+			Labels = new ReadOnlyCollection<string> (labels);
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageLabelsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageLabelsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="labels">The message labels.</param>
+		/// <param name="modseq">The modification sequence value.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="labels"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageLabelsChangedEventArgs (int index, IList<string> labels, ulong modseq) : base (index)
+		{
+			if (labels == null)
+				throw new ArgumentNullException (nameof (labels));
+
+			Labels = new ReadOnlyCollection<string> (labels);
+			ModSeq = modseq;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageLabelsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageLabelsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="uid">The unique id of the message.</param>
+		/// <param name="labels">The message labels.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="labels"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageLabelsChangedEventArgs (int index, UniqueId uid, IList<string> labels) : base (index, uid)
+		{
+			if (labels == null)
+				throw new ArgumentNullException (nameof (labels));
+
+			Labels = new ReadOnlyCollection<string> (labels);
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageLabelsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageLabelsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="uid">The unique id of the message.</param>
+		/// <param name="labels">The message labels.</param>
+		/// <param name="modseq">The modification sequence value.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="labels"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		public MessageLabelsChangedEventArgs (int index, UniqueId uid, IList<string> labels, ulong modseq) : base (index, uid)
+		{
+			if (labels == null)
+				throw new ArgumentNullException (nameof (labels));
+
+			Labels = new ReadOnlyCollection<string> (labels);
+			ModSeq = modseq;
+		}
+
+		/// <summary>
+		/// Gets the updated labels.
+		/// </summary>
+		/// <remarks>
+		/// Gets the updated labels.
+		/// </remarks>
+		/// <value>The updated labels.</value>
+		public IList<string> Labels {
+			get; internal set;
+		}
+
+		/// <summary>
+		/// Gets the updated mod-sequence value of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// Gets the updated mod-sequence value of the message, if available.
+		/// </remarks>
+		/// <value>The mod-sequence value.</value>
+		public ulong? ModSeq {
+			get; internal set;
+		}
+	}
+}

src/MailKit/MessageNotFoundException.cs

@@ -0,0 +1,92 @@
+//
+// MessageNotFoundException.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+#if SERIALIZABLE
+using System.Security;
+using System.Runtime.Serialization;
+#endif
+
+namespace MailKit {
+	/// <summary>
+	/// The exception that is thrown when a message (or body part) could not be found.
+	/// </summary>
+	/// <remarks>
+	/// This exception is thrown by methods such as
+	/// <a href="Overload_MailKit_IMailFolder_GetMessage.htm">IMailFolder.GetMessage</a>,
+	/// <a href="Overload_MailKit_IMailFolder_GetBodyPart.htm">IMailFolder.GetBodyPart</a>, or
+	/// <a href="Overload_MailKit_IMailFolder_GetStream.htm">IMailFolder.GetStream</a>
+	/// when the server's response does not contain the message, body part, or stream data requested.
+	/// </remarks>
+#if SERIALIZABLE
+	[Serializable]
+#endif
+	public class MessageNotFoundException : Exception
+	{
+#if SERIALIZABLE
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageNotFoundException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Deserializes a <see cref="MessageNotFoundException"/>.
+		/// </remarks>
+		/// <param name="info">The serialization info.</param>
+		/// <param name="context">The streaming context.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="info"/> is <c>null</c>.
+		/// </exception>
+		[SecuritySafeCritical]
+		protected MessageNotFoundException (SerializationInfo info, StreamingContext context) : base (info, context)
+		{
+		}
+#endif
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageNotFoundException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageNotFoundException"/>.
+		/// </remarks>
+		/// <param name="message">The error message.</param>
+		/// <param name="innerException">The inner exception.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// </exception>
+		public MessageNotFoundException (string message, Exception innerException) : base (message, innerException)
+		{
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageNotFoundException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageNotFoundException"/>.
+		/// </remarks>
+		/// <param name="message">The error message.</param>
+		public MessageNotFoundException (string message) : base (message)
+		{
+		}
+	}
+}

src/MailKit/MessageSentEventArgs.cs

@@ -0,0 +1,87 @@
+//
+// MessageSentEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+using MimeKit;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args used when a message is successfully sent.
+	/// </summary>
+	/// <remarks>
+	/// Event args used when message is successfully sent.
+	/// </remarks>
+	public class MessageSentEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageSentEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageSentEventArgs"/>.
+		/// </remarks>
+		/// <param name="message">The message that was just sent.</param>
+		/// <param name="response">The response from the server.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="message"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="response"/> is <c>null</c>.</para>
+		/// </exception>
+		public MessageSentEventArgs (MimeMessage message, string response)
+		{
+			if (message == null)
+				throw new ArgumentNullException (nameof (message));
+
+			if (response == null)
+				throw new ArgumentNullException (nameof (response));
+
+			Message = message;
+			Response = response;
+		}
+
+		/// <summary>
+		/// Get the message that was just sent.
+		/// </summary>
+		/// <remarks>
+		/// Gets the message that was just sent.
+		/// </remarks>
+		/// <value>The message.</value>
+		public MimeMessage Message {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the server's response.
+		/// </summary>
+		/// <remarks>
+		/// Gets the server's response.
+		/// </remarks>
+		/// <value>The response.</value>
+		public string Response {
+			get; private set;
+		}
+	}
+}

src/MailKit/MessageSorter.cs

@@ -0,0 +1,295 @@
+//
+// MessageSorter.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Linq;
+using System.Collections.Generic;
+
+using MimeKit;
+using MailKit.Search;
+
+namespace MailKit {
+	/// <summary>
+	/// Routines for sorting messages.
+	/// </summary>
+	/// <remarks>
+	/// Routines for sorting messages.
+	/// </remarks>
+	public static class MessageSorter
+	{
+		class MessageComparer<T> : IComparer<T> where T : IMessageSummary
+		{
+			readonly IList<OrderBy> orderBy;
+
+			public MessageComparer (IList<OrderBy> orderBy)
+			{
+				this.orderBy = orderBy;
+			}
+
+			#region IComparer implementation
+
+			static int CompareDisplayNames (InternetAddressList list1, InternetAddressList list2)
+			{
+				var m1 = list1.Mailboxes.GetEnumerator ();
+				var m2 = list2.Mailboxes.GetEnumerator ();
+				bool n1 = m1.MoveNext ();
+				bool n2 = m2.MoveNext ();
+
+				while (n1 && n2) {
+					var name1 = m1.Current.Name ?? string.Empty;
+					var name2 = m2.Current.Name ?? string.Empty;
+					int cmp;
+
+					if ((cmp = string.Compare (name1, name2, StringComparison.OrdinalIgnoreCase)) != 0)
+						return cmp;
+
+					n1 = m1.MoveNext ();
+					n2 = m2.MoveNext ();
+				}
+
+				return n1 ? 1 : (n2 ? -1  : 0);
+			}
+
+			static int CompareMailboxAddresses (InternetAddressList list1, InternetAddressList list2)
+			{
+				var m1 = list1.Mailboxes.GetEnumerator ();
+				var m2 = list2.Mailboxes.GetEnumerator ();
+				bool n1 = m1.MoveNext ();
+				bool n2 = m2.MoveNext ();
+
+				while (n1 && n2) {
+					int cmp;
+
+					if ((cmp = string.Compare (m1.Current.Address, m2.Current.Address, StringComparison.OrdinalIgnoreCase)) != 0)
+						return cmp;
+
+					n1 = m1.MoveNext ();
+					n2 = m2.MoveNext ();
+				}
+
+				return n1 ? 1 : (n2 ? -1  : 0);
+			}
+
+			public int Compare (T x, T y)
+			{
+				int cmp = 0;
+
+				for (int i = 0; i < orderBy.Count; i++) {
+					switch (orderBy[i].Type) {
+					case OrderByType.Annotation:
+						var annotation = (OrderByAnnotation) orderBy[i];
+
+						var xannotation = x.Annotations?.FirstOrDefault (a => a.Entry == annotation.Entry);
+						var yannotation = y.Annotations?.FirstOrDefault (a => a.Entry == annotation.Entry);
+
+						var xvalue = xannotation?.Properties[annotation.Attribute] ?? string.Empty;
+						var yvalue = yannotation?.Properties[annotation.Attribute] ?? string.Empty;
+
+						cmp = string.Compare (xvalue, yvalue, StringComparison.OrdinalIgnoreCase);
+						break;
+					case OrderByType.Arrival:
+						cmp = x.Index.CompareTo (y.Index);
+						break;
+					case OrderByType.Cc:
+						cmp = CompareMailboxAddresses (x.Envelope.Cc, y.Envelope.Cc);
+						break;
+					case OrderByType.Date:
+						cmp = x.Date.CompareTo (y.Date);
+						break;
+					case OrderByType.DisplayFrom:
+						cmp = CompareDisplayNames (x.Envelope.From, y.Envelope.From);
+						break;
+					case OrderByType.From:
+						cmp = CompareMailboxAddresses (x.Envelope.From, y.Envelope.From);
+						break;
+					case OrderByType.ModSeq:
+						var xmodseq = x.ModSeq ?? 0;
+						var ymodseq = y.ModSeq ?? 0;
+
+						cmp = xmodseq.CompareTo (ymodseq);
+						break;
+					case OrderByType.Size:
+						var xsize = x.Size ?? 0;
+						var ysize = y.Size ?? 0;
+
+						cmp = xsize.CompareTo (ysize);
+						break;
+					case OrderByType.Subject:
+						var xsubject = x.Envelope.Subject ?? string.Empty;
+						var ysubject = y.Envelope.Subject ?? string.Empty;
+
+						cmp = string.Compare (xsubject, ysubject, StringComparison.OrdinalIgnoreCase);
+						break;
+					case OrderByType.DisplayTo:
+						cmp = CompareDisplayNames (x.Envelope.To, y.Envelope.To);
+						break;
+					case OrderByType.To:
+						cmp = CompareMailboxAddresses (x.Envelope.To, y.Envelope.To);
+						break;
+					}
+
+					if (cmp == 0)
+						continue;
+
+					return orderBy[i].Order == SortOrder.Descending ? cmp * -1 : cmp;
+				}
+
+				return cmp;
+			}
+
+			#endregion
+		}
+
+		static MessageSummaryItems GetMessageSummaryItems (IList<OrderBy> orderBy)
+		{
+			var items = MessageSummaryItems.None;
+
+			for (int i = 0; i < orderBy.Count; i++) {
+				switch (orderBy[i].Type) {
+				case OrderByType.Annotation:
+					items |= MessageSummaryItems.Annotations;
+					break;
+				case OrderByType.Arrival:
+					break;
+				case OrderByType.Cc:
+				case OrderByType.Date:
+				case OrderByType.DisplayFrom:
+				case OrderByType.DisplayTo:
+				case OrderByType.From:
+				case OrderByType.Subject:
+				case OrderByType.To:
+					items |= MessageSummaryItems.Envelope;
+					break;
+				case OrderByType.ModSeq:
+					items |= MessageSummaryItems.ModSeq;
+					break;
+				case OrderByType.Size:
+					items |= MessageSummaryItems.Size;
+					break;
+				}
+			}
+
+			return items;
+		}
+
+		/// <summary>
+		/// Sorts the messages by the specified ordering.
+		/// </summary>
+		/// <remarks>
+		/// Sorts the messages by the specified ordering.
+		/// </remarks>
+		/// <returns>The sorted messages.</returns>
+		/// <typeparam name="T">The message items must implement the <see cref="IMessageSummary"/> interface.</typeparam>
+		/// <param name="messages">The messages to sort.</param>
+		/// <param name="orderBy">The sort ordering.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="messages"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="orderBy"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <para><paramref name="messages"/> contains one or more items that is missing information needed for sorting.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="orderBy"/> is an empty list.</para>
+		/// </exception>
+		public static IList<T> Sort<T> (this IEnumerable<T> messages, IList<OrderBy> orderBy) where T : IMessageSummary
+		{
+			if (messages == null)
+				throw new ArgumentNullException (nameof (messages));
+
+			if (orderBy == null)
+				throw new ArgumentNullException (nameof (orderBy));
+
+			if (orderBy.Count == 0)
+				throw new ArgumentException ("No sort order provided.", nameof (orderBy));
+
+			var requiredFields = GetMessageSummaryItems (orderBy);
+			var list = new List<T> ();
+
+			foreach (var message in messages) {
+				if ((message.Fields & requiredFields) != requiredFields)
+					throw new ArgumentException ("One or more messages is missing information needed for sorting.", nameof (messages));
+
+				list.Add (message);
+			}
+
+			if (list.Count < 2)
+				return list;
+
+			var comparer = new MessageComparer<T> (orderBy);
+
+			list.Sort (comparer);
+
+			return list;
+		}
+
+		/// <summary>
+		/// Sorts the messages by the specified ordering.
+		/// </summary>
+		/// <remarks>
+		/// Sorts the messages by the specified ordering.
+		/// </remarks>
+		/// <returns>The sorted messages.</returns>
+		/// <typeparam name="T">The message items must implement the <see cref="IMessageSummary"/> interface.</typeparam>
+		/// <param name="messages">The messages to sort.</param>
+		/// <param name="orderBy">The sort ordering.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="messages"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="orderBy"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <para><paramref name="messages"/> contains one or more items that is missing information needed for sorting.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="orderBy"/> is an empty list.</para>
+		/// </exception>
+		public static void Sort<T> (this List<T> messages, IList<OrderBy> orderBy) where T : IMessageSummary
+		{
+			if (messages == null)
+				throw new ArgumentNullException (nameof (messages));
+
+			if (orderBy == null)
+				throw new ArgumentNullException (nameof (orderBy));
+
+			if (orderBy.Count == 0)
+				throw new ArgumentException ("No sort order provided.", nameof (orderBy));
+
+			var requiredFields = GetMessageSummaryItems (orderBy);
+
+			for (int i = 0; i < messages.Count; i++) {
+				if ((messages[i].Fields & requiredFields) != requiredFields)
+					throw new ArgumentException ("One or more messages is missing information needed for sorting.", nameof (messages));
+			}
+
+			if (messages.Count < 2)
+				return;
+
+			var comparer = new MessageComparer<T> (orderBy);
+
+			messages.Sort (comparer);
+		}
+	}
+}

src/MailKit/MessageSummary.cs

@@ -0,0 +1,758 @@
+//
+// MessageSummary.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Linq;
+using System.Collections.Generic;
+
+using MimeKit;
+using MimeKit.Utils;
+
+namespace MailKit {
+	/// <summary>
+	/// A summary of a message.
+	/// </summary>
+	/// <remarks>
+	/// <para>The <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a> and
+	/// <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a> methods
+	/// return lists of <see cref="IMessageSummary"/> items.</para>
+	/// <para>The properties of the <see cref="MessageSummary"/> that will be available
+	/// depend on the <see cref="MessageSummaryItems"/> passed to the aformentioned method.</para>
+	/// </remarks>
+	public class MessageSummary : IMessageSummary
+	{
+		int threadableReplyDepth = -1;
+		string normalizedSubject;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageSummary"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageSummary"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is negative.
+		/// </exception>
+		public MessageSummary (int index)
+		{
+			if (index < 0)
+				throw new ArgumentOutOfRangeException (nameof (index));
+
+			Keywords = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
+			Index = index;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageSummary"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageSummary"/>.
+		/// </remarks>
+		/// <param name="folder">The folder that the message belongs to.</param>
+		/// <param name="index">The message index.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="folder"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is negative.
+		/// </exception>
+		public MessageSummary (IMailFolder folder, int index) : this (index)
+		{
+			if (folder == null)
+				throw new ArgumentNullException (nameof (folder));
+
+			Folder = folder;
+		}
+
+		void UpdateThreadableSubject ()
+		{
+			if (normalizedSubject != null)
+				return;
+
+			if (Envelope?.Subject != null) {
+				normalizedSubject = MessageThreader.GetThreadableSubject (Envelope.Subject, out threadableReplyDepth);
+			} else {
+				normalizedSubject = string.Empty;
+				threadableReplyDepth = 0;
+			}
+		}
+
+		/// <summary>
+		/// Get the folder that the message belongs to.
+		/// </summary>
+		/// <remarks>
+		/// Gets the folder that the message belongs to, if available.
+		/// </remarks>
+		/// <value>The folder.</value>
+		public IMailFolder Folder {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get a bitmask of fields that have been populated.
+		/// </summary>
+		/// <remarks>
+		/// Gets a bitmask of fields that have been populated.
+		/// </remarks>
+		/// <value>The fields that have been populated.</value>
+		public MessageSummaryItems Fields {
+			get; internal set;
+		}
+
+		/// <summary>
+		/// Gets the body structure of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>The body will be one of <see cref="BodyPartText"/>,
+		/// <see cref="BodyPartMessage"/>, <see cref="BodyPartBasic"/>,
+		/// or <see cref="BodyPartMultipart"/>.</para>
+		/// <para>This property will only be set if either the
+		/// <see cref="MessageSummaryItems.Body"/> flag or the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The body structure of the message.</value>
+		public BodyPart Body {
+			get; set;
+		}
+
+		static BodyPart GetMultipartRelatedRoot (BodyPartMultipart related)
+		{
+			string start = related.ContentType.Parameters["start"];
+			string contentId;
+
+			if (start == null)
+				return related.BodyParts.Count > 0 ? related.BodyParts[0] : null;
+
+			if ((contentId = MimeUtils.EnumerateReferences (start).FirstOrDefault ()) == null)
+				contentId = start;
+
+			var cid = new Uri (string.Format ("cid:{0}", contentId));
+
+			for (int i = 0; i < related.BodyParts.Count; i++) {
+				var basic = related.BodyParts[i] as BodyPartBasic;
+
+				if (basic != null && (basic.ContentId == contentId || basic.ContentLocation == cid))
+					return basic;
+
+				var multipart = related.BodyParts[i] as BodyPartMultipart;
+
+				if (multipart != null && multipart.ContentLocation == cid)
+					return multipart;
+			}
+
+			return null;
+		}
+
+		static bool TryGetMultipartAlternativeBody (BodyPartMultipart multipart, bool html, out BodyPartText body)
+		{
+			// walk the multipart/alternative children backwards from greatest level of faithfulness to the least faithful
+			for (int i = multipart.BodyParts.Count - 1; i >= 0; i--) {
+				var multi = multipart.BodyParts[i] as BodyPartMultipart;
+				BodyPartText text = null;
+
+				if (multi != null) {
+					if (multi.ContentType.IsMimeType ("multipart", "related")) {
+						text = GetMultipartRelatedRoot (multi) as BodyPartText;
+					} else if (multi.ContentType.IsMimeType ("multipart", "alternative")) {
+						// Note: nested multipart/alternatives make no sense... yet here we are.
+						if (TryGetMultipartAlternativeBody (multi, html, out body))
+							return true;
+					}
+				} else {
+					text = multipart.BodyParts[i] as BodyPartText;
+				}
+
+				if (text != null && (html ? text.IsHtml : text.IsPlain)) {
+					body = text;
+					return true;
+				}
+			}
+
+			body = null;
+
+			return false;
+		}
+
+		static bool TryGetMessageBody (BodyPartMultipart multipart, bool html, out BodyPartText body)
+		{
+			BodyPartMultipart multi;
+			BodyPartText text;
+
+			if (multipart.ContentType.IsMimeType ("multipart", "alternative"))
+				return TryGetMultipartAlternativeBody (multipart, html, out body);
+
+			if (!multipart.ContentType.IsMimeType ("multipart", "related")) {
+				// Note: This is probably a multipart/mixed... and if not, we can still treat it like it is.
+				for (int i = 0; i < multipart.BodyParts.Count; i++) {
+					multi = multipart.BodyParts[i] as BodyPartMultipart;
+
+					// descend into nested multiparts, if there are any...
+					if (multi != null) {
+						if (TryGetMessageBody (multi, html, out body))
+							return true;
+
+						// The text body should never come after a multipart.
+						break;
+					}
+
+					text = multipart.BodyParts[i] as BodyPartText;
+
+					// Look for the first non-attachment text part (realistically, the body text will
+					// preceed any attachments, but I'm not sure we can rely on that assumption).
+					if (text != null && !text.IsAttachment) {
+						if (html ? text.IsHtml : text.IsPlain) {
+							body = text;
+							return true;
+						}
+
+						// Note: the first text/* part in a multipart/mixed is the text body.
+						// If it's not in the format we're looking for, then it doesn't exist.
+						break;
+					}
+				}
+			} else {
+				// Note: If the multipart/related root document is HTML, then this is the droid we are looking for.
+				var root = GetMultipartRelatedRoot (multipart);
+
+				text = root as BodyPartText;
+
+				if (text != null) {
+					body = (html ? text.IsHtml : text.IsPlain) ? text : null;
+					return body != null;
+				}
+
+				// maybe the root is another multipart (like multipart/alternative)?
+				multi = root as BodyPartMultipart;
+
+				if (multi != null)
+					return TryGetMessageBody (multi, html, out body);
+			}
+
+			body = null;
+
+			return false;
+		}
+
+		/// <summary>
+		/// Gets the text body part of the message if it exists.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the <c>text/plain</c> body part of the message.</para>
+		/// <para>This property will only be usable if the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadBodyParts"/>
+		/// </example>
+		/// <value>The text body if it exists; otherwise, <c>null</c>.</value>
+		public BodyPartText TextBody {
+			get {
+				var multipart = Body as BodyPartMultipart;
+
+				if (multipart != null) {
+					BodyPartText plain;
+
+					if (TryGetMessageBody (multipart, false, out plain))
+						return plain;
+				} else {
+					var text = Body as BodyPartText;
+
+					if (text != null && text.IsPlain)
+						return text;
+				}
+
+				return null;
+			}
+		}
+
+		/// <summary>
+		/// Gets the html body part of the message if it exists.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the <c>text/html</c> body part of the message.</para>
+		/// <para>This property will only be usable if the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The html body if it exists; otherwise, <c>null</c>.</value>
+		public BodyPartText HtmlBody {
+			get {
+				var multipart = Body as BodyPartMultipart;
+
+				if (multipart != null) {
+					BodyPartText html;
+
+					if (TryGetMessageBody (multipart, true, out html))
+						return html;
+				} else {
+					var text = Body as BodyPartText;
+
+					if (text != null && text.IsHtml)
+						return text;
+				}
+
+				return null;
+			}
+		}
+
+		static IEnumerable<BodyPartBasic> EnumerateBodyParts (BodyPart entity, bool attachmentsOnly)
+		{
+			if (entity == null)
+				yield break;
+
+			var multipart = entity as BodyPartMultipart;
+
+			if (multipart != null) {
+				foreach (var subpart in multipart.BodyParts) {
+					foreach (var part in EnumerateBodyParts (subpart, attachmentsOnly))
+						yield return part;
+				}
+
+				yield break;
+			}
+
+			var basic = (BodyPartBasic) entity;
+
+			if (attachmentsOnly && !basic.IsAttachment)
+				yield break;
+
+			yield return basic;
+		}
+
+		/// <summary>
+		/// Gets the body parts of the message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Traverses over the <see cref="Body"/>, enumerating all of the
+		/// <see cref="BodyPartBasic"/> objects.</para>
+		/// <para>This property will only be usable if either the
+		/// <see cref="MessageSummaryItems.Body"/> flag or the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The body parts.</value>
+		public IEnumerable<BodyPartBasic> BodyParts {
+			get { return EnumerateBodyParts (Body, false); }
+		}
+
+		/// <summary>
+		/// Gets the attachments.
+		/// </summary>
+		/// <remarks>
+		/// <para>Traverses over the <see cref="Body"/>, enumerating all of the
+		/// <see cref="BodyPartBasic"/> objects that have a <c>Content-Disposition</c>
+		/// header set to <c>"attachment"</c>.</para>
+		/// <para>This property will only be usable if the
+		/// <see cref="MessageSummaryItems.BodyStructure"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadBodyParts"/>
+		/// </example>
+		/// <value>The attachments.</value>
+		public IEnumerable<BodyPartBasic> Attachments {
+			get { return EnumerateBodyParts (Body, true); }
+		}
+
+		/// <summary>
+		/// Gets the preview text of the message.
+		/// </summary>
+		/// <remarks>
+		/// <para>The preview text is a short snippet of the beginning of the message
+		/// text, typically shown in a mail client's message list to provide the user
+		/// with a sense of what the message is about.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.PreviewText"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The preview text.</value>
+		public string PreviewText {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the envelope of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>The envelope of a message contains information such as the
+		/// date the message was sent, the subject of the message,
+		/// the sender of the message, who the message was sent to,
+		/// which message(s) the message may be in reply to,
+		/// and the message id.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Envelope"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The envelope of the message.</value>
+		public Envelope Envelope {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the normalized subject.
+		/// </summary>
+		/// <remarks>
+		/// A normalized Subject header value where prefixes such as
+		/// "Re:", "Re[#]:", etc have been pruned.
+		/// </remarks>
+		/// <value>The normalized subject.</value>
+		public string NormalizedSubject {
+			get {
+				UpdateThreadableSubject ();
+
+				return normalizedSubject;
+			}
+		}
+
+		/// <summary>
+		/// Gets whether or not the message is a reply.
+		/// </summary>
+		/// <remarks>
+		/// This value should be based on whether the message subject contained any "Re:" or "Fwd:" prefixes.
+		/// </remarks>
+		/// <value><c>true</c> if the message is a reply; otherwise, <c>false</c>.</value>
+		public bool IsReply {
+			get {
+				UpdateThreadableSubject ();
+
+				return threadableReplyDepth != 0;
+			}
+		}
+
+		/// <summary>
+		/// Gets the Date header value.
+		/// </summary>
+		/// <remarks>
+		/// Gets the Date header value. If the Date header is not present, the arrival date is used.
+		/// If neither are known, <see cref="System.DateTimeOffset.MinValue"/> is returned.
+		/// </remarks>
+		/// <value>The date.</value>
+		public DateTimeOffset Date {
+			get { return Envelope?.Date ?? InternalDate ?? DateTimeOffset.MinValue; }
+		}
+
+		/// <summary>
+		/// Gets the message flags, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the message flags, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Flags"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The message flags.</value>
+		public MessageFlags? Flags {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the user-defined message flags, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the user-defined message flags, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Flags"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The user-defined message flags.</value>
+		public HashSet<string> Keywords {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the user-defined message flags, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the user-defined message flags, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Flags"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The user-defined message flags.</value>
+		[Obsolete ("Use Keywords instead.")]
+		public HashSet<string> UserFlags {
+			get { return Keywords; }
+			set { Keywords = value; }
+		}
+
+		/// <summary>
+		/// Gets the message annotations, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the message annotations, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Annotations"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The message annotations.</value>
+		public IList<Annotation> Annotations {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the list of headers, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the list of headers, if available.</para>
+		/// <para>This property will only be set if <see cref="MessageSummaryItems.Headers"/>
+		/// is specified in a call to one of the
+		/// <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods or specific headers are requested via a one of the Fetch or FetchAsync methods
+		/// that accept list of specific headers to request for each message such as
+		/// <see cref="IMailFolder.Fetch(System.Collections.Generic.IList&lt;UniqueId&gt;,MessageSummaryItems,System.Collections.Generic.IEnumerable&lt;MimeKit.HeaderId&gt;,System.Threading.CancellationToken)"/>.
+		/// </para>
+		/// </remarks>
+		/// <value>The list of headers.</value>
+		public HeaderList Headers {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the internal date of the message (i.e. the "received" date), if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the internal date of the message (i.e. the "received" date), if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.InternalDate"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The internal date of the message.</value>
+		public DateTimeOffset? InternalDate {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the size of the message, in bytes, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the size of the message, in bytes, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.Size"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The size of the message.</value>
+		public uint? Size {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the mod-sequence value for the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the mod-sequence value for the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.ModSeq"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The mod-sequence value.</value>
+		public ulong? ModSeq {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the message-ids that the message references, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the message-ids that the message references, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.References"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The references.</value>
+		public MessageIdList References {
+			get; set;
+		}
+
+		/// <summary>
+		/// Get the globally unique identifier for the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the globally unique identifier of the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.EmailId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// <note type="info">This property maps to the <c>EMAILID</c> value defined in the
+		/// <a href="https://tools.ietf.org/html/rfc8474">OBJECTID</a> extension.</note>
+		/// </remarks>
+		/// <value>The globally unique message identifier.</value>
+		public string EmailId {
+			get; set;
+		}
+
+		/// <summary>
+		/// Get the globally unique identifier for the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the globally unique identifier of the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.EmailId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// <note type="info">This property maps to the <c>EMAILID</c> value defined in the
+		/// <a href="https://tools.ietf.org/html/rfc8474">OBJECTID</a> extension.</note>
+		/// </remarks>
+		/// <value>The globally unique message identifier.</value>
+		[Obsolete ("Use EmailId instead.")]
+		public string Id {
+			get { return EmailId; }
+		}
+
+		/// <summary>
+		/// Get the globally unique thread identifier for the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the globally unique thread identifier for the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.ThreadId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// <note type="info">This property maps to the <c>THREADID</c> value defined in the
+		/// <a href="https://tools.ietf.org/html/rfc8474">OBJECTID</a> extension.</note>
+		/// </remarks>
+		/// <value>The globally unique thread identifier.</value>
+		public string ThreadId {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the unique identifier of the message, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the unique identifier of the message, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.UniqueId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The uid of the message.</value>
+		public UniqueId UniqueId {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the index of the message.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the index of the message.</para>
+		/// <para>This property is always set.</para>
+		/// </remarks>
+		/// <value>The index of the message.</value>
+		public int Index {
+			get; internal set;
+		}
+
+		#region GMail extension properties
+
+		/// <summary>
+		/// Gets the GMail message identifier, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the GMail message identifier, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.GMailMessageId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The GMail message identifier.</value>
+		public ulong? GMailMessageId {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the GMail thread identifier, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the GMail thread identifier, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.GMailThreadId"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The GMail thread identifier.</value>
+		public ulong? GMailThreadId {
+			get; set;
+		}
+
+		/// <summary>
+		/// Gets the list of GMail labels, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the list of GMail labels, if available.</para>
+		/// <para>This property will only be set if the
+		/// <see cref="MessageSummaryItems.GMailLabels"/> flag is passed to
+		/// one of the <a href="Overload_MailKit_IMailFolder_Fetch.htm">Fetch</a>
+		/// or <a href="Overload_MailKit_IMailFolder_FetchAsync.htm">FetchAsync</a>
+		/// methods.</para>
+		/// </remarks>
+		/// <value>The GMail labels.</value>
+		public IList<string> GMailLabels {
+			get; set;
+		}
+
+		#endregion
+	}
+}

src/MailKit/MessageSummaryFetchedEventArgs.cs

@@ -0,0 +1,67 @@
+//
+// MessageSummaryFetchedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args used when a message summary has been fetched from a folder.
+	/// </summary>
+	/// <remarks>
+	/// Event args used when a message summary has been fetched from a folder.
+	/// </remarks>
+	public class MessageSummaryFetchedEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageSummaryFetchedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageSummaryFetchedEventArgs"/>
+		/// </remarks>
+		/// <param name="message">The message summary.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="message"/> is <c>null</c>.
+		/// </exception>
+		public MessageSummaryFetchedEventArgs (IMessageSummary message)
+		{
+			if (message == null)
+				throw new ArgumentNullException (nameof (message));
+
+			Message = message;
+		}
+
+		/// <summary>
+		/// Get the message summary.
+		/// </summary>
+		/// <remarks>
+		/// Gets the message summary.
+		/// </remarks>
+		/// <value>The message summary.</value>
+		public IMessageSummary Message {
+			get; private set;
+		}
+	}
+}

src/MailKit/MessageSummaryItems.cs

@@ -0,0 +1,222 @@
+//
+// FetchFlags.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// A bitfield of <see cref="MessageSummary"/> fields.
+	/// </summary>
+	/// <remarks>
+	/// <see cref="MessageSummaryItems"/> are used to specify which properties
+	/// of <see cref="MessageSummary"/> should be populated by calls to
+	/// <see cref="IMailFolder.Fetch(System.Collections.Generic.IList&lt;UniqueId&gt;, MessageSummaryItems, System.Threading.CancellationToken)"/>,
+	/// <see cref="IMailFolder.Fetch(System.Collections.Generic.IList&lt;int&gt;, MessageSummaryItems, System.Threading.CancellationToken)"/>, or
+	/// <see cref="IMailFolder.Fetch(int, int, MessageSummaryItems, System.Threading.CancellationToken)"/>.
+	/// </remarks>
+	[Flags]
+	public enum MessageSummaryItems {
+		/// <summary>
+		/// Don't fetch any summary items.
+		/// </summary>
+		None           = 0,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.Annotations"/>.</para>
+		/// <para>Fetches all <c>ANNOATION</c> values as defined in
+		/// <a href="https://tools.ietf.org/html/rfc5257">rfc5257</a>.</para>
+		/// </summary>
+		Annotations    = 1 << 0,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.Body"/>.</para>
+		/// <para>Fetches the <c>BODY</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// <note type="note">Unlike <see cref="BodyStructure"/>, <c>Body</c> will not populate the
+		/// <see cref="BodyPart.ContentType"/> parameters nor will it populate the
+		/// <see cref="BodyPartBasic.ContentDisposition"/>, <see cref="BodyPartBasic.ContentLanguage"/>
+		/// or <see cref="BodyPartBasic.ContentLocation"/> properties of each <see cref="BodyPartBasic"/>
+		/// body part. This makes <c>Body</c> far less useful than <c>BodyStructure</c> especially when
+		/// it is desirable to determine whether or not a body part is an attachment.</note>
+		/// </summary>
+		Body           = 1 << 1,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.Body"/> (but with more details than <see cref="Body"/>).</para>
+		/// <para>Fetches the <c>BODYSTRUCTURE</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// <note type="note">Unlike <see cref="Body"/>, <c>BodyStructure</c> will also populate the
+		/// <see cref="BodyPart.ContentType"/> parameters as well as the
+		/// <see cref="BodyPartBasic.ContentDisposition"/>, <see cref="BodyPartBasic.ContentLanguage"/>
+		/// and <see cref="BodyPartBasic.ContentLocation"/> properties of each <see cref="BodyPartBasic"/>
+		/// body part. The <c>Content-Disposition</c> information is especially important when trying to
+		/// determine whether or not a body part is an attachment, for example.</note>
+		/// </summary>
+		BodyStructure  = 1 << 2,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.Envelope"/>.</para>
+		/// <para>Fetches the <c>ENVELOPE</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// </summary>
+		Envelope       = 1 << 3,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.Flags"/>.</para>
+		/// <para>Fetches the <c>FLAGS</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// </summary>
+		Flags          = 1 << 4,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.InternalDate"/>.</para>
+		/// <para>Fetches the <c>INTERNALDATE</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// </summary>
+		InternalDate   = 1 << 5,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.Size"/>.</para>
+		/// <para>Fetches the <c>RFC822.SIZE</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// </summary>
+		Size           = 1 << 6,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.ModSeq"/>.</para>
+		/// <para>Fetches the <c>MODSEQ</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc4551">rfc4551</a>.</para>
+		/// </summary>
+		ModSeq         = 1 << 7,
+
+		/// <summary>
+		/// Fetch the <see cref="IMessageSummary.References"/>.
+		/// </summary>
+		References     = 1 << 8,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.UniqueId"/>.</para>
+		/// <para>Fetches the <c>UID</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// </summary>
+		UniqueId       = 1 << 9,
+
+		/// <summary>
+		/// <para></para>Fetch the <see cref="IMessageSummary.EmailId"/>.
+		/// <para>Fetches the <c>EMAILID</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc8474">rfc8474</a>.</para>
+		/// </summary>
+		EmailId        = 1 << 10,
+
+		/// <summary>
+		/// <para></para>Fetch the <see cref="IMessageSummary.EmailId"/>.
+		/// <para>Fetches the <c>EMAILID</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc8474">rfc8474</a>.</para>
+		/// </summary>
+		[Obsolete ("Use EmailId instead.")]
+		Id             = EmailId,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.ThreadId"/>.</para>
+		/// <para>Fetches the <c>THREADID</c> value as defined in
+		/// <a href="https://tools.ietf.org/html/rfc8474">rfc8474</a>.</para>
+		/// </summary>
+		ThreadId       = 1 << 11,
+
+		#region GMail extension items
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.GMailMessageId"/>.</para>
+		/// <para>Fetches the <c>X-GM-MSGID</c> value as defined in Google's
+		/// <a href="https://developers.google.com/gmail/imap/imap-extensions">IMAP extensions</a>
+		/// documentation.</para>
+		/// </summary>
+		GMailMessageId = 1 << 12,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.GMailThreadId"/>.</para>
+		/// <para>Fetches the <c>X-GM-THRID</c> value as defined in Google's
+		/// <a href="https://developers.google.com/gmail/imap/imap-extensions">IMAP extensions</a>
+		/// documentation.</para>
+		/// </summary>
+		GMailThreadId  = 1 << 13,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.GMailLabels"/>.</para>
+		/// <para>Fetches the <c>X-GM-LABELS</c> value as defined in Google's
+		/// <a href="https://developers.google.com/gmail/imap/imap-extensions">IMAP extensions</a>
+		/// documentation.</para>
+		/// </summary>
+		GMailLabels    = 1 << 14,
+
+		#endregion
+
+		/// <summary>
+		/// <para>Fetch the the complete list of <see cref="IMessageSummary.Headers"/> for each message.</para>
+		/// </summary>
+		Headers        = 1 << 15,
+
+		/// <summary>
+		/// <para>Fetch the <see cref="IMessageSummary.PreviewText"/>.</para>
+		/// <note type="note">This property is quite expensive to calculate because it is not an
+		/// item that is cached on the IMAP server. Instead, MailKit must download a hunk of the
+		/// message body so that it can decode and parse it in order to generate a meaningful
+		/// text snippet. This usually involves downloading the first 512 bytes for <c>text/plain</c>
+		/// message bodies and the first 16 kilobytes for <c>text/html</c> message bodies. If a
+		/// message contains both a <c>text/plain</c> body and a <c>text/html</c> body, then the
+		/// <c>text/plain</c> content is used in order to reduce network traffic.</note>
+		/// </summary>
+		PreviewText    = 1 << 16,
+
+		#region Macros
+
+		/// <summary>
+		/// <para>A macro for fetching the <see cref="Envelope"/>, <see cref="Flags"/>,
+		/// <see cref="InternalDate"/>, and <see cref="Size"/> values.</para>
+		/// <para>This macro maps to the equivalent <c>ALL</c> macro as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// </summary>
+		All           = Envelope | Flags | InternalDate | Size,
+
+		/// <summary>
+		/// <para>A macro for fetching the <see cref="Flags"/>, <see cref="InternalDate"/>, and
+		/// <see cref="Size"/> values.</para>
+		/// <para>This macro maps to the equivalent <c>FAST</c> macro as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// </summary>
+		Fast          = Flags | InternalDate | Size,
+
+		/// <summary>
+		/// <para>A macro for fetching the <see cref="Body"/>, <see cref="Envelope"/>,
+		/// <see cref="Flags"/>, <see cref="InternalDate"/>, and <see cref="Size"/> values.</para>
+		/// <para>This macro maps to the equivalent <c>FULL</c> macro as defined in
+		/// <a href="https://tools.ietf.org/html/rfc3501">rfc3501</a>.</para>
+		/// </summary>
+		Full          = Body | Envelope | Flags| InternalDate | Size,
+
+		#endregion
+	}
+}

src/MailKit/MessageThread.cs

@@ -0,0 +1,108 @@
+//
+// MessageThread.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Collections.Generic;
+
+namespace MailKit {
+	/// <summary>
+	/// A message thread.
+	/// </summary>
+	/// <remarks>
+	/// A message thread.
+	/// </remarks>
+	public class MessageThread
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageThread"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new message thread node.
+		/// </remarks>
+		/// <param name="uid">The unique identifier of the message.</param>
+		public MessageThread (UniqueId? uid)
+		{
+			Children = new List<MessageThread> ();
+			UniqueId = uid;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageThread"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new message thread node.
+		/// </remarks>
+		/// <param name="message">The message summary.</param>
+		public MessageThread (IMessageSummary message)
+		{
+			Children = new List<MessageThread> ();
+			if (message != null && message.UniqueId.IsValid)
+				UniqueId = message?.UniqueId;
+			Message = message;
+		}
+
+		/// <summary>
+		/// Gets the message summary, if available.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the message summary, if available.</para>
+		/// <note type="note">This property will only ever be set if the <see cref="MessageThread"/>
+		/// was created by the <see cref="MessageThreader"/>. <see cref="MessageThread"/>s that are
+		/// created by any of the
+		/// <a href="Overload_MailKit_Net_Imap_ImapFolder_Thread.htm">Thread</a> or
+		/// <a href="Overload_MailKit_Net_Imap_ImapFolder_ThreadAsync.htm">ThreadAsync</a>
+		/// methods will always be <c>null</c>.</note>
+		/// </remarks>
+		/// <value>The message summary.</value>
+		public IMessageSummary Message {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the unique identifier of the message.
+		/// </summary>
+		/// <remarks>
+		/// The unique identifier may be <c>null</c> if the message is missing from the
+		/// <see cref="IMailFolder"/> or from the list of messages provided to the
+		/// <see cref="MessageThreader"/>.
+		/// </remarks>
+		/// <value>The unique identifier.</value>
+		public UniqueId? UniqueId {
+			// FIXME: this shouldn't be a nullable since we can just use UniqueId.Invalid
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the children.
+		/// </summary>
+		/// <remarks>
+		/// Each child represents a reply to the message referenced by <see cref="UniqueId"/>.
+		/// </remarks>
+		/// <value>The children.</value>
+		public IList<MessageThread> Children {
+			get; private set;
+		}
+	}
+}

src/MailKit/MessageThreader.cs

@@ -0,0 +1,603 @@
+//
+// MessageThreader.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+using System.Collections.Generic;
+
+using MimeKit;
+using MimeKit.Utils;
+
+using MailKit.Search;
+
+namespace MailKit {
+	/// <summary>
+	/// Threads messages according to the algorithms defined in rfc5256.
+	/// </summary>
+	/// <remarks>
+	/// Threads messages according to the algorithms defined in rfc5256.
+	/// </remarks>
+	public static class MessageThreader
+	{
+		internal class ThreadableNode : IMessageSummary
+		{
+			public readonly List<ThreadableNode> Children = new List<ThreadableNode> ();
+			public IMessageSummary Message;
+			public ThreadableNode Parent;
+
+			public ThreadableNode (IMessageSummary message)
+			{
+				Message = message;
+			}
+
+			public bool HasParent {
+				get { return Parent != null; }
+			}
+
+			public bool HasChildren {
+				get { return Children.Count > 0; }
+			}
+
+			public IMailFolder Folder => null;
+
+			public MessageSummaryItems Fields {
+				get { return MessageSummaryItems.UniqueId | MessageSummaryItems.Envelope | MessageSummaryItems.ModSeq | MessageSummaryItems.Size; }
+			}
+
+			public BodyPart Body => null;
+
+			public BodyPartText TextBody => null;
+
+			public BodyPartText HtmlBody => null;
+
+			public IEnumerable<BodyPartBasic> BodyParts => null;
+
+			public IEnumerable<BodyPartBasic> Attachments => null;
+
+			public string PreviewText => null;
+
+			public Envelope Envelope {
+				get { return Message != null ? Message.Envelope : Children[0].Envelope; }
+			}
+
+			public string NormalizedSubject {
+				get { return Message != null ? Message.NormalizedSubject : Children[0].NormalizedSubject; }
+			}
+
+			public DateTimeOffset Date {
+				get { return Message != null ? Message.Date : Children[0].Date; }
+			}
+
+			public bool IsReply {
+				get { return Message != null && Message.IsReply; }
+			}
+
+			public MessageFlags? Flags => null;
+
+			public HashSet<string> Keywords => null;
+
+			[Obsolete]
+			public HashSet<string> UserFlags => null;
+
+			public IList<Annotation> Annotations {
+				get { return Message != null ? Message.Annotations : Children[0].Annotations; }
+			}
+
+			public HeaderList Headers => null;
+
+			public DateTimeOffset? InternalDate => null;
+
+			public uint? Size {
+				get { return Message != null ? Message.Size : Children[0].Size; }
+			}
+
+			public ulong? ModSeq {
+				get { return Message != null ? Message.ModSeq : Children[0].ModSeq; }
+			}
+
+			public MessageIdList References {
+				get { return Message != null ? Message.References : Children[0].References; }
+			}
+
+			public string EmailId => null;
+
+			[Obsolete]
+			public string Id => null;
+
+			public string ThreadId => null;
+
+			public UniqueId UniqueId {
+				get { return Message != null ? Message.UniqueId : Children[0].UniqueId; }
+			}
+
+			public int Index {
+				get { return Message != null ? Message.Index : Children[0].Index; }
+			}
+
+			public ulong? GMailMessageId => null;
+
+			public ulong? GMailThreadId => null;
+
+			public IList<string> GMailLabels => null;
+		}
+
+		static IDictionary<string, ThreadableNode> CreateIdTable (IEnumerable<IMessageSummary> messages)
+		{
+			var ids = new Dictionary<string, ThreadableNode> (StringComparer.OrdinalIgnoreCase);
+			ThreadableNode node;
+
+			foreach (var message in messages) {
+				if (message.Envelope == null)
+					throw new ArgumentException ("One or more messages is missing information needed for threading.", nameof (messages));
+
+				var id = message.Envelope.MessageId;
+
+				if (string.IsNullOrEmpty (id))
+					id = MimeUtils.GenerateMessageId ();
+
+				if (ids.TryGetValue (id, out node)) {
+					if (node.Message == null) {
+						// a previously processed message referenced this message
+						node.Message = message;
+					} else {
+						// a duplicate message-id, just create a dummy id and use that
+						id = MimeUtils.GenerateMessageId ();
+						node = null;
+					}
+				}
+
+				if (node == null) {
+					// create a new ThreadContainer for this message and add it to ids
+					node = new ThreadableNode (message);
+					ids.Add (id, node);
+				}
+
+				ThreadableNode parent = null;
+				foreach (var reference in message.References) {
+					ThreadableNode referenced;
+
+					if (!ids.TryGetValue (reference, out referenced)) {
+						// create a dummy container for the referenced message
+						referenced = new ThreadableNode (null);
+						ids.Add (reference, referenced);
+					}
+
+					// chain up the references, disallowing loops
+					if (parent != null && referenced.Parent == null && parent != referenced && !parent.Children.Contains (referenced)) {
+						parent.Children.Add (referenced);
+						referenced.Parent = parent;
+					}
+
+					parent = referenced;
+				}
+
+				// don't allow loops
+				if (parent != null && (parent == node || node.Children.Contains (parent)))
+					parent = null;
+
+				if (node.HasParent) {
+					// unlink from our old parent
+					node.Parent.Children.Remove (node);
+					node.Parent = null;
+				}
+
+				if (parent != null) {
+					// add it as a child of our new parent
+					parent.Children.Add (node);
+					node.Parent = parent;
+				}
+			}
+
+			return ids;
+		}
+
+		static ThreadableNode CreateRoot (IDictionary<string, ThreadableNode> ids)
+		{
+			var root = new ThreadableNode (null);
+
+			foreach (var message in ids.Values) {
+				if (message.Parent == null)
+					root.Children.Add (message);
+			}
+
+			return root;
+		}
+
+		static void PruneEmptyContainers (ThreadableNode root)
+		{
+			for (int i = 0; i < root.Children.Count; i++) {
+				var node = root.Children[i];
+
+				if (node.Message == null && node.Children.Count == 0) {
+					// this is an empty container with no children, nuke it.
+					root.Children.RemoveAt (i);
+					i--;
+				} else if (node.Message == null && node.HasChildren && (node.HasParent || node.Children.Count == 1)) {
+					// If the Container has no Message, but does have children, remove this container but promote
+					// its children to this level (that is, splice them in to the current child list.)
+					//
+					// Do not promote the children if doing so would promote them to the root set -- unless there
+					// is only one child, in which case, do.
+					root.Children.RemoveAt (i);
+
+					for (int j = 0; j < node.Children.Count; j++) {
+						node.Children[j].Parent = node.Parent;
+						root.Children.Add (node.Children[j]);
+					}
+
+					node.Children.Clear ();
+					i--;
+				} else if (node.HasChildren) {
+					PruneEmptyContainers (node);
+				}
+			}
+		}
+
+		static void GroupBySubject (ThreadableNode root)
+		{
+			var subjects = new Dictionary<string, ThreadableNode> (StringComparer.OrdinalIgnoreCase);
+			ThreadableNode match;
+			int count = 0;
+
+			for (int i = 0; i < root.Children.Count; i++) {
+				var current = root.Children[i];
+				var subject = current.NormalizedSubject;
+
+				// don't thread messages with empty subjects
+				if (string.IsNullOrEmpty (subject))
+					continue;
+
+				if (!subjects.TryGetValue (subject, out match) ||
+					(current.Message == null && match.Message != null) ||
+					(match.Message != null && match.Message.IsReply &&
+						current.Message != null && !current.Message.IsReply)) {
+					subjects[subject] = current;
+					count++;
+				}
+			}
+
+			if (count == 0)
+				return;
+
+			for (int i = 0; i < root.Children.Count; i++) {
+				var current = root.Children[i];
+				var subject = current.NormalizedSubject;
+
+				// don't thread messages with empty subjects
+				if (string.IsNullOrEmpty (subject))
+					continue;
+
+				match = subjects[subject];
+
+				if (match == current)
+					continue;
+
+				// remove the second message with the same subject
+				root.Children.RemoveAt (i--);
+
+				// group these messages together...
+				if (match.Message == null && current.Message == null) {
+					// If both messages are dummies, append the current message's children
+					// to the children of the message in the subject table (the children of
+					// both messages become siblings), and then delete the current message.
+					match.Children.AddRange (current.Children);
+				} else if (match.Message == null && current.Message != null) {
+					// If the message in the subject table is a dummy and the current message
+					// is not, make the current message a child of the message in the subject
+					// table (a sibling of its children).
+					match.Children.Add (current);
+				} else if (current.Message.IsReply && !match.Message.IsReply) {
+					// If the current message is a reply or forward and the message in the
+					// subject table is not, make the current message a child of the message
+					// in the subject table (a sibling of its children).
+					match.Children.Add (current);
+				} else {
+					// Otherwise, create a new dummy message and make both the current message
+					// and the message in the subject table children of the dummy. Then replace
+					// the message in the subject table with the dummy message.
+
+					// Note: if we re-use the node already in the subject table and the root, then
+					// we won't have to insert the new dummy node at the matched node's location
+					var dummy = match;
+
+					// clone the message already in the subject table
+					match = new ThreadableNode (dummy.Message);
+					match.Children.AddRange (dummy.Children);
+
+					// empty out the old match node (aka the new dummy node)
+					dummy.Children.Clear ();
+					dummy.Message = null;
+
+					// now add both messages to the dummy
+					dummy.Children.Add (match);
+					dummy.Children.Add (current);
+				}
+			}
+		}
+
+		static void GetThreads (ThreadableNode root, IList<MessageThread> threads, IList<OrderBy> orderBy)
+		{
+			root.Children.Sort (orderBy);
+
+			for (int i = 0; i < root.Children.Count; i++) {
+				var message = root.Children[i].Message;
+				var thread = new MessageThread (message);
+
+				GetThreads (root.Children[i], thread.Children, orderBy);
+				threads.Add (thread);
+			}
+		}
+
+		static IList<MessageThread> ThreadByReferences (IEnumerable<IMessageSummary> messages, IList<OrderBy> orderBy)
+		{
+			var threads = new List<MessageThread> ();
+			var ids = CreateIdTable (messages);
+			var root = CreateRoot (ids);
+
+			PruneEmptyContainers (root);
+			GroupBySubject (root);
+
+			GetThreads (root, threads, orderBy);
+
+			return threads;
+		}
+
+		static IList<MessageThread> ThreadBySubject (IEnumerable<IMessageSummary> messages, IList<OrderBy> orderBy)
+		{
+			var threads = new List<MessageThread> ();
+			var root = new ThreadableNode (null);
+
+			foreach (var message in messages) {
+				if (message.Envelope == null)
+					throw new ArgumentException ("One or more messages is missing information needed for threading.", nameof (messages));
+
+				var node = new ThreadableNode (message);
+
+				root.Children.Add (node);
+			}
+
+			GroupBySubject (root);
+
+			GetThreads (root, threads, orderBy);
+
+			return threads;
+		}
+
+		/// <summary>
+		/// Thread the messages according to the specified threading algorithm.
+		/// </summary>
+		/// <remarks>
+		/// Thread the messages according to the specified threading algorithm.
+		/// </remarks>
+		/// <returns>The threaded messages.</returns>
+		/// <param name="messages">The messages.</param>
+		/// <param name="algorithm">The threading algorithm.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="messages"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="algorithm"/> is not a valid threading algorithm.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="messages"/> contains one or more items that is missing information needed for threading.
+		/// </exception>
+		public static IList<MessageThread> Thread (this IEnumerable<IMessageSummary> messages, ThreadingAlgorithm algorithm)
+		{
+			return Thread (messages, algorithm, new [] { OrderBy.Arrival });
+		}
+
+		/// <summary>
+		/// Threads the messages according to the specified threading algorithm
+		/// and sorts the resulting threads by the specified ordering.
+		/// </summary>
+		/// <remarks>
+		/// Threads the messages according to the specified threading algorithm
+		/// and sorts the resulting threads by the specified ordering.
+		/// </remarks>
+		/// <returns>The threaded messages.</returns>
+		/// <param name="messages">The messages.</param>
+		/// <param name="algorithm">The threading algorithm.</param>
+		/// <param name="orderBy">The requested sort ordering.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="messages"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="orderBy"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="algorithm"/> is not a valid threading algorithm.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <para><paramref name="messages"/> contains one or more items that is missing information needed for threading or sorting.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="orderBy"/> is an empty list.</para>
+		/// </exception>
+		public static IList<MessageThread> Thread (this IEnumerable<IMessageSummary> messages, ThreadingAlgorithm algorithm, IList<OrderBy> orderBy)
+		{
+			if (messages == null)
+				throw new ArgumentNullException (nameof (messages));
+
+			if (orderBy == null)
+				throw new ArgumentNullException (nameof (orderBy));
+
+			if (orderBy.Count == 0)
+				throw new ArgumentException ("No sort order provided.", nameof (orderBy));
+
+			switch (algorithm) {
+			case ThreadingAlgorithm.OrderedSubject: return ThreadBySubject (messages, orderBy);
+			case ThreadingAlgorithm.References: return ThreadByReferences (messages, orderBy);
+			default: throw new ArgumentOutOfRangeException (nameof (algorithm));
+			}
+		}
+
+		static bool IsForward (string subject, int index)
+		{
+			return (subject[index] == 'F' || subject[index] == 'f') &&
+				(subject[index + 1] == 'W' || subject[index + 1] == 'w') &&
+				(subject[index + 2] == 'D' || subject[index + 2] == 'd') &&
+				subject[index + 3] == ':';
+		}
+
+		static bool IsReply (string subject, int index)
+		{
+			return (subject[index] == 'R' || subject[index] == 'r') &&
+				(subject[index + 1] == 'E' || subject[index + 1] == 'e');
+		}
+
+		static void SkipWhiteSpace (string subject, ref int index)
+		{
+			while (index < subject.Length && char.IsWhiteSpace (subject[index]))
+				index++;
+		}
+
+		static bool IsMailingListName (char c)
+		{
+			return c == '-' || c == '_' || char.IsLetterOrDigit (c);
+		}
+
+		static void SkipMailingListName (string subject, ref int index)
+		{
+			while (index < subject.Length && IsMailingListName (subject[index]))
+				index++;
+		}
+
+		static bool SkipDigits (string subject, ref int index, out int value)
+		{
+			int startIndex = index;
+
+			value = 0;
+
+			while (index < subject.Length && char.IsDigit (subject[index])) {
+				value = (value * 10) + (subject[index] - '0');
+				index++;
+			}
+
+			return index > startIndex;
+		}
+
+		/// <summary>
+		/// Gets the threadable subject.
+		/// </summary>
+		/// <remarks>
+		/// Gets the threadable subject.
+		/// </remarks>
+		/// <returns>The threadable subject.</returns>
+		/// <param name="subject">The Subject header value.</param>
+		/// <param name="replyDepth">The reply depth.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="subject"/> is <c>null</c>.
+		/// </exception>
+		public static string GetThreadableSubject (string subject, out int replyDepth)
+		{
+			if (subject == null)
+				throw new ArgumentNullException (nameof (subject));
+
+			replyDepth = 0;
+
+			int endIndex = subject.Length;
+			int startIndex = 0;
+			int index, count;
+			int left;
+
+			do {
+				SkipWhiteSpace (subject, ref startIndex);
+				index = startIndex;
+
+				if ((left = (endIndex - index)) < 3)
+					break;
+
+				if (left >= 4 && IsForward (subject, index)) {
+					// skip over the "Fwd:" prefix
+					startIndex = index + 4;
+					replyDepth++;
+					continue;
+				}
+
+				if (IsReply (subject, index)) {
+					if (subject[index + 2] == ':') {
+						// skip over the "Re:" prefix
+						startIndex = index + 3;
+						replyDepth++;
+						continue;
+					}
+
+					if (subject[index + 2] == '[' || subject[index + 2] == '(') {
+						char close = subject[index + 2] == '[' ? ']' : ')';
+
+						// skip over "Re[" or "Re("
+						index += 3;
+
+						// if this is followed by "###]:" or "###):", then it's a condensed "Re:"
+						if (SkipDigits (subject, ref index, out count) && (endIndex - index) >= 2 &&
+							subject[index] == close && subject[index + 1] == ':') {
+							startIndex = index + 2;
+							replyDepth += count;
+							continue;
+						}
+					}
+				} else if (subject[index] == '[' && char.IsLetterOrDigit (subject[index + 1])) {
+					// possibly a mailing-list prefix
+					index += 2;
+
+					SkipMailingListName (subject, ref index);
+
+					if ((endIndex - index) >= 1 && subject[index] == ']') {
+						startIndex = index + 1;
+						continue;
+					}
+				}
+
+				break;
+			} while (true);
+
+			// trim trailing whitespace
+			while (endIndex > 0 && char.IsWhiteSpace (subject[endIndex - 1]))
+				endIndex--;
+
+			// canonicalize the remainder of the subject, condensing multiple spaces into 1
+			var builder = new StringBuilder ();
+			bool lwsp = false;
+
+			for (int i = startIndex; i < endIndex; i++) {
+				if (char.IsWhiteSpace (subject[i])) {
+					if (!lwsp) {
+						builder.Append (' ');
+						lwsp = true;
+					}
+				} else {
+					builder.Append (subject[i]);
+					lwsp = false;
+				}
+			}
+
+			var canonicalized = builder.ToString ();
+
+			if (canonicalized.Equals ("(no subject)", StringComparison.OrdinalIgnoreCase))
+				canonicalized = string.Empty;
+
+			return canonicalized;
+		}
+	}
+}

src/MailKit/MessagesVanishedEventArgs.cs

@@ -0,0 +1,79 @@
+//
+// MessagesVanishedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args used when a message vanishes from a folder.
+	/// </summary>
+	/// <remarks>
+	/// Event args used when a message vanishes from a folder.
+	/// </remarks>
+	public class MessagesVanishedEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessagesVanishedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessagesVanishedEventArgs"/>.
+		/// </remarks>
+		/// <param name="uids">The list of unique identifiers.</param>
+		/// <param name="earlier">If set to <c>true</c>, the messages vanished in the past as opposed to just now.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="uids"/> is <c>null</c>.
+		/// </exception>
+		public MessagesVanishedEventArgs (IList<UniqueId> uids, bool earlier)
+		{
+			UniqueIds = new ReadOnlyCollection<UniqueId> (uids);
+			Earlier = earlier;
+		}
+
+		/// <summary>
+		/// Gets the unique identifiers of the messages that vanished.
+		/// </summary>
+		/// <remarks>
+		/// Gets the unique identifiers of the messages that vanished.
+		/// </remarks>
+		/// <value>The unique identifiers.</value>
+		public IList<UniqueId> UniqueIds {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets whether the messages vanished in the past as opposed to just now.
+		/// </summary>
+		/// <remarks>
+		/// Gets whether the messages vanished in the past as opposed to just now.
+		/// </remarks>
+		/// <value><c>true</c> if the messages vanished earlier; otherwise, <c>false</c>.</value>
+		public bool Earlier {
+			get; private set;
+		}
+	}
+}

src/MailKit/Metadata.cs

@@ -0,0 +1,74 @@
+//
+// Metadata.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit {
+	/// <summary>
+	/// A metadata tag and value.
+	/// </summary>
+	/// <remarks>
+	/// A metadata tag and value.
+	/// </remarks>
+	public class Metadata
+	{
+		internal string EncodedName;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Metadata"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="Metadata"/>.
+		/// </remarks>
+		/// <param name="tag">The metadata tag.</param>
+		/// <param name="value">The metadata value.</param>
+		public Metadata (MetadataTag tag, string value)
+		{
+			Value = value;
+			Tag = tag;
+		}
+
+		/// <summary>
+		/// Gets the metadata tag.
+		/// </summary>
+		/// <remarks>
+		/// Gets the metadata tag.
+		/// </remarks>
+		/// <value>The metadata tag.</value>
+		public MetadataTag Tag {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the metadata value.
+		/// </summary>
+		/// <remarks>
+		/// Gets the metadata value.
+		/// </remarks>
+		/// <value>The metadata value.</value>
+		public string Value {
+			get; private set;
+		}
+	}
+}

src/MailKit/MetadataChangedEventArgs.cs

@@ -0,0 +1,67 @@
+//
+// MetadataChangedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// Event args used when a metadata changes.
+	/// </summary>
+	/// <remarks>
+	/// Event args used when a metadata changes.
+	/// </remarks>
+	public class MetadataChangedEventArgs : EventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.MetadataChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MetadataChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="metadata">The metadata that changed.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="metadata"/> is <c>null</c>.
+		/// </exception>
+		public MetadataChangedEventArgs (Metadata metadata)
+		{
+			if (metadata == null)
+				throw new ArgumentNullException (nameof (metadata));
+
+			Metadata = metadata;
+		}
+
+		/// <summary>
+		/// Get the metadata that changed.
+		/// </summary>
+		/// <remarks>
+		/// Gets the metadata that changed.
+		/// </remarks>
+		/// <value>The metadata.</value>
+		public Metadata Metadata {
+			get; private set;
+		}
+	}
+}

src/MailKit/MetadataCollection.cs

@@ -0,0 +1,59 @@
+//
+// MetadataCollection.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Collections.Generic;
+
+namespace MailKit {
+	/// <summary>
+	/// A collection of metadata.
+	/// </summary>
+	/// <remarks>
+	/// A collection of metadata.
+	/// </remarks>
+	public class MetadataCollection : List<Metadata>
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MetadataCollection"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MetadataCollection"/>
+		/// </remarks>
+		public MetadataCollection ()
+		{
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MetadataCollection"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MetadataCollection"/>.
+		/// </remarks>
+		/// <param name="collection">A collection of metadata.</param>
+		public MetadataCollection (IEnumerable<Metadata> collection) : base (collection)
+		{
+		}
+	}
+}

src/MailKit/MetadataOptions.cs

@@ -0,0 +1,108 @@
+//
+// MetadataOptions.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit
+{
+	/// <summary>
+	/// A set of options to use when requesting metadata.
+	/// </summary>
+	/// <remarks>
+	/// A set of options to use when requesting metadata.
+	/// </remarks>
+	public class MetadataOptions
+	{
+		int depth;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.MetadataOptions"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new set of options to use when requesting metadata.
+		/// </remarks>
+		public MetadataOptions ()
+		{
+		}
+
+		/// <summary>
+		/// Get or set the depth.
+		/// </summary>
+		/// <remarks>
+		/// <para>When the <see cref="Depth"/> option is specified, it extends the list of metadata tag
+		/// values returned by the GetMetadata() call. For each <see cref="MetadataTag"/> specified in the
+		/// the GetMetadata() call, the method returns the value of the specified metadata tag (if it exists),
+		/// plus all metadata tags below the specified entry up to the specified depth.</para>
+		/// <para>Three values are allowed for <see cref="Depth"/>:</para>
+		/// <para><c>0</c> - no entries below the specified metadata tag are returned.</para>
+		/// <para><c>1</c> - only entries immediately below the specified metadata tag are returned.</para>
+		/// <para><see cref="System.Int32.MaxValue"/> - all entries below the specified metadata tag are returned.</para>
+		/// <para>Thus, a depth of <c>1</c> for a tag entry of <c>"/a"</c> will match <c>"/a"</c> as well as its children
+		/// entries (e.g., <c>"/a/b"</c>), but will not match grandchildren entries (e.g., <c>"/a/b/c"</c>).</para>
+		/// <para>If the Depth option is not specified, this is the same as specifying <c>0</c>.</para>
+		/// </remarks>
+		/// <value>The depth.</value>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="value"/> is out of range.
+		/// </exception>
+		public int Depth {
+			get { return depth; }
+			set {
+				if (!(value == 0 || value == 1 || value == int.MaxValue))
+					throw new ArgumentOutOfRangeException (nameof (value));
+
+				depth = value;
+			}
+		}
+
+		/// <summary>
+		/// Get or set the max size of the metadata tags to request.
+		/// </summary>
+		/// <remarks>
+		/// When specified, the <see cref="MaxSize"/> property is used to filter the metadata tags
+		/// returned by the GetMetadata() call to only those with a value shorter than the max size
+		/// specified.
+		/// </remarks>
+		/// <value>The size of the max.</value>
+		public uint? MaxSize {
+			get; set;
+		}
+
+		/// <summary>
+		/// Get the length of the longest metadata value.
+		/// </summary>
+		/// <remarks>
+		/// If the <see cref="MaxSize"/> property is specified, once the GetMetadata() call returns,
+		/// the <see cref="LongEntries"/> property will be set to the length of the longest metadata
+		/// value that exceeded the <see cref="MaxSize"/> limit, otherwise a value of <c>0</c> will
+		/// be set.
+		/// </remarks>
+		/// <value>The length of the longest metadata value that exceeded the max size.</value>
+		public uint LongEntries {
+			get; set;
+		}
+	}
+}

src/MailKit/MetadataTag.cs

@@ -0,0 +1,156 @@
+//
+// MetadataTag.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit {
+	/// <summary>
+	/// A metadata tag.
+	/// </summary>
+	/// <remarks>
+	/// A metadata tag.
+	/// </remarks>
+	public struct MetadataTag
+	{
+		/// <summary>
+		/// A metadata tag for specifying the contact information for the server administrator.
+		/// </summary>
+		/// <remarks>
+		/// Used to get the contact information of the administrator on a
+		/// <see cref="IMailStore"/>.
+		/// </remarks>
+		public static readonly MetadataTag SharedAdmin = new MetadataTag ("/shared/admin");
+
+		/// <summary>
+		/// A metadata tag for private comments.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set a private comment on a <see cref="IMailFolder"/>.
+		/// </remarks>
+		public static readonly MetadataTag PrivateComment = new MetadataTag ("/private/comment");
+
+		/// <summary>
+		/// A metadata tag for shared comments.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set a shared comment on a <see cref="IMailStore"/>
+		/// or <see cref="IMailFolder"/>.
+		/// </remarks>
+		public static readonly MetadataTag SharedComment = new MetadataTag ("/shared/comment");
+
+		/// <summary>
+		/// A metadata tag for specifying the special use of a folder.
+		/// </summary>
+		/// <remarks>
+		/// Used to get or set the special use of a <see cref="IMailFolder"/>.
+		/// </remarks>
+		public static readonly MetadataTag PrivateSpecialUse = new MetadataTag ("/private/specialuse");
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MetadataTag"/> struct.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MetadataTag"/>.
+		/// </remarks>
+		/// <param name="id">The metadata tag identifier.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="id"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="id"/> is an empty string.
+		/// </exception>
+		public MetadataTag (string id)
+		{
+			if (id == null)
+				throw new ArgumentNullException (nameof (id));
+
+			if (id.Length == 0)
+				throw new ArgumentException ("A metadata tag identifier cannot be empty.");
+
+			Id = id;
+		}
+
+		/// <summary>
+		/// Get the metadata tag identifier.
+		/// </summary>
+		/// <remarks>
+		/// Gets the metadata tag identifier.
+		/// </remarks>
+		/// <value>The metadata tag identifier.</value>
+		public string Id {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Determine whether the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.MetadataTag"/>.
+		/// </summary>
+		/// <remarks>
+		/// Determines whether the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.MetadataTag"/>.
+		/// </remarks>
+		/// <param name="obj">The <see cref="System.Object"/> to compare with the current <see cref="MailKit.MetadataTag"/>.</param>
+		/// <returns><c>true</c> if the specified <see cref="System.Object"/> is equal to the current
+		/// <see cref="MailKit.MetadataTag"/>; otherwise, <c>false</c>.</returns>
+		public override bool Equals (object obj)
+		{
+			return obj is MetadataTag && ((MetadataTag) obj).Id == Id;
+		}
+
+		/// <summary>
+		/// Serves as a hash function for a <see cref="MailKit.MetadataTag"/> object.
+		/// </summary>
+		/// <remarks>
+		/// Serves as a hash function for a <see cref="MailKit.MetadataTag"/> object.
+		/// </remarks>
+		/// <returns>A hash code for this instance that is suitable for use in hashing algorithms and data structures such as a hash table.</returns>
+		public override int GetHashCode ()
+		{
+			return Id.GetHashCode ();
+		}
+
+		/// <summary>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.MetadataTag"/>.
+		/// </summary>
+		/// <remarks>
+		/// Returns a <see cref="System.String"/> that represents the current <see cref="MailKit.MetadataTag"/>.
+		/// </remarks>
+		/// <returns>A <see cref="System.String"/> that represents the current <see cref="MailKit.MetadataTag"/>.</returns>
+		public override string ToString ()
+		{
+			return Id;
+		}
+
+		internal static MetadataTag Create (string id)
+		{
+			switch (id) {
+			case "/shared/admin":       return SharedAdmin;
+			case "/private/comment":    return PrivateComment;
+			case "/shared/comment":     return SharedComment;
+			case "/private/specialuse": return PrivateSpecialUse;
+			default: return new MetadataTag (id);
+			}
+		}
+	}
+}

src/MailKit/ModSeqChangedEventArgs.cs

@@ -0,0 +1,86 @@
+//
+// ModSeqChangedEventArgs.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit
+{
+	/// <summary>
+	/// Event args for the <see cref="IMailFolder.ModSeqChanged"/> event.
+	/// </summary>
+	/// <remarks>
+	/// Event args for the <see cref="IMailFolder.ModSeqChanged"/> event.
+	/// </remarks>
+	public class ModSeqChangedEventArgs : MessageEventArgs
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.ModSeqChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ModSeqChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		internal ModSeqChangedEventArgs (int index) : base (index)
+		{
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="modseq">The modification sequence value.</param>
+		public ModSeqChangedEventArgs (int index, ulong modseq) : base (index)
+		{
+			ModSeq = modseq;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.ModSeqChangedEventArgs"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ModSeqChangedEventArgs"/>.
+		/// </remarks>
+		/// <param name="index">The message index.</param>
+		/// <param name="uid">The unique id of the message.</param>
+		/// <param name="modseq">The modification sequence value.</param>
+		public ModSeqChangedEventArgs (int index, UniqueId uid, ulong modseq) : base (index, uid)
+		{
+			ModSeq = modseq;
+		}
+
+		/// <summary>
+		/// Gets the updated mod-sequence value of the message.
+		/// </summary>
+		/// <remarks>
+		/// Gets the updated mod-sequence value of the message.
+		/// </remarks>
+		/// <value>The mod-sequence value.</value>
+		public ulong ModSeq {
+			get; internal set;
+		}
+	}
+}

src/MailKit/Net/Imap/AsyncImapClient.cs

@@ -0,0 +1,965 @@
+//
+// AsyncImapClient.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.IO;
+using System.Net;
+using System.Text;
+using System.Threading;
+using System.Net.Sockets;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+using MailKit.Security;
+
+namespace MailKit.Net.Imap
+{
+	public partial class ImapClient
+	{
+		/// <summary>
+		/// Asynchronously enable compression over the IMAP connection.
+		/// </summary>
+		/// <remarks>
+		/// <para>Asynchronously enables compression over the IMAP connection.</para>
+		/// <para>If the IMAP server supports the <see cref="ImapCapabilities.Compress"/> extension,
+		/// it is possible at any point after connecting to enable compression to reduce network
+		/// bandwidth usage. Ideally, this method should be called before authenticating.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Compression must be enabled before a folder has been selected.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the COMPRESS extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the COMPRESS command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public Task CompressAsync (CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return CompressAsync (true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously enable the QRESYNC feature.
+		/// </summary>
+		/// <remarks>
+		/// <para>Enables the <a href="https://tools.ietf.org/html/rfc5162">QRESYNC</a> feature.</para>
+		/// <para>The QRESYNC extension improves resynchronization performance of folders by
+		/// querying the IMAP server for a list of changes when the folder is opened using the
+		/// <see cref="ImapFolder.Open(FolderAccess,uint,ulong,System.Collections.Generic.IList&lt;UniqueId&gt;,System.Threading.CancellationToken)"/>
+		/// method.</para>
+		/// <para>If this feature is enabled, the <see cref="MailFolder.MessageExpunged"/> event is replaced
+		/// with the <see cref="MailFolder.MessagesVanished"/> event.</para>
+		/// <para>This method needs to be called immediately after calling one of the
+		/// <a href="Overload_MailKit_Net_Imap_ImapClient_Authenticate.htm">Authenticate</a> methods, before
+		/// opening any folders.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Quick resynchronization needs to be enabled before selecting a folder.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the QRESYNC extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the ENABLE command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public override Task EnableQuickResyncAsync (CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return EnableQuickResyncAsync (true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously enable the UTF8=ACCEPT extension.
+		/// </summary>
+		/// <remarks>
+		/// Enables the <a href="https://tools.ietf.org/html/rfc6855">UTF8=ACCEPT</a> extension.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// UTF8=ACCEPT needs to be enabled before selecting a folder.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the UTF8=ACCEPT extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the ENABLE command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public Task EnableUTF8Async (CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return EnableUTF8Async (true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously identify the client implementation to the server and obtain the server implementation details.
+		/// </summary>
+		/// <remarks>
+		/// <para>Passes along the client implementation details to the server while also obtaining implementation
+		/// details from the server.</para>
+		/// <para>If the <paramref name="clientImplementation"/> is <c>null</c> or no properties have been set, no
+		/// identifying information will be sent to the server.</para>
+		/// <note type="security">
+		/// <para>Security Implications</para>
+		/// <para>This command has the danger of violating the privacy of users if misused. Clients should
+		/// notify users that they send the ID command.</para>
+		/// <para>It is highly desirable that implementations provide a method of disabling ID support, perhaps by
+		/// not calling this method at all, or by passing <c>null</c> as the <paramref name="clientImplementation"/>
+		/// argument.</para>
+		/// <para>Implementors must exercise extreme care in adding properties to the <paramref name="clientImplementation"/>.
+		/// Some properties, such as a processor ID number, Ethernet address, or other unique (or mostly unique) identifier
+		/// would allow tracking of users in ways that violate user privacy expectations and may also make it easier for
+		/// attackers to exploit security holes in the client.</para>
+		/// </note>
+		/// </remarks>
+		/// <returns>The implementation details of the server if available; otherwise, <c>null</c>.</returns>
+		/// <param name="clientImplementation">The client implementation.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the ID extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the ID command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public Task<ImapImplementation> IdentifyAsync (ImapImplementation clientImplementation, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return IdentifyAsync (clientImplementation, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously authenticate using the specified SASL mechanism.
+		/// </summary>
+		/// <remarks>
+		/// <para>Authenticates using the specified SASL mechanism.</para>
+		/// <para>For a list of available SASL authentication mechanisms supported by the server,
+		/// check the <see cref="AuthenticationMechanisms"/> property after the service has been
+		/// connected.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="mechanism">The SASL mechanism.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="mechanism"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// The <see cref="ImapClient"/> is already authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="MailKit.Security.AuthenticationException">
+		/// Authentication using the supplied credentials has failed.
+		/// </exception>
+		/// <exception cref="MailKit.Security.SaslException">
+		/// A SASL authentication error occurred.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// An IMAP command failed.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public override Task AuthenticateAsync (SaslMechanism mechanism, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return AuthenticateAsync (mechanism, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously authenticate using the supplied credentials.
+		/// </summary>
+		/// <remarks>
+		/// <para>If the IMAP server supports one or more SASL authentication mechanisms,
+		/// then the SASL mechanisms that both the client and server support are tried
+		/// in order of greatest security to weakest security. Once a SASL
+		/// authentication mechanism is found that both client and server support,
+		/// the credentials are used to authenticate.</para>
+		/// <para>If the server does not support SASL or if no common SASL mechanisms
+		/// can be found, then LOGIN command is used as a fallback.</para>
+		/// <note type="tip">To prevent the usage of certain authentication mechanisms,
+		/// simply remove them from the <see cref="AuthenticationMechanisms"/> hash set
+		/// before calling this method.</note>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="encoding">The text encoding to use for the user's credentials.</param>
+		/// <param name="credentials">The user's credentials.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="encoding"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="credentials"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// The <see cref="ImapClient"/> is already authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="MailKit.Security.AuthenticationException">
+		/// Authentication using the supplied credentials has failed.
+		/// </exception>
+		/// <exception cref="MailKit.Security.SaslException">
+		/// A SASL authentication error occurred.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// An IMAP command failed.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public override Task AuthenticateAsync (Encoding encoding, ICredentials credentials, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return AuthenticateAsync (encoding, credentials, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously establish a connection to the specified IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// <para>Establishes a connection to the specified IMAP or IMAP/S server.</para>
+		/// <para>If the <paramref name="port"/> has a value of <c>0</c>, then the
+		/// <paramref name="options"/> parameter is used to determine the default port to
+		/// connect to. The default port used with <see cref="SecureSocketOptions.SslOnConnect"/>
+		/// is <c>993</c>. All other values will use a default port of <c>143</c>.</para>
+		/// <para>If the <paramref name="options"/> has a value of
+		/// <see cref="SecureSocketOptions.Auto"/>, then the <paramref name="port"/> is used
+		/// to determine the default security options. If the <paramref name="port"/> has a value
+		/// of <c>993</c>, then the default options used will be
+		/// <see cref="SecureSocketOptions.SslOnConnect"/>. All other values will use
+		/// <see cref="SecureSocketOptions.StartTlsWhenAvailable"/>.</para>
+		/// <para>Once a connection is established, properties such as
+		/// <see cref="AuthenticationMechanisms"/> and <see cref="Capabilities"/> will be
+		/// populated.</para>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadMessages"/>
+		/// </example>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="host">The host name to connect to.</param>
+		/// <param name="port">The port to connect to. If the specified port is <c>0</c>, then the default port will be used.</param>
+		/// <param name="options">The secure socket options to when connecting.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="host"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="port"/> is not between <c>0</c> and <c>65535</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// The <paramref name="host"/> is a zero-length string.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// The <see cref="ImapClient"/> is already connected.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <paramref name="options"/> was set to
+		/// <see cref="MailKit.Security.SecureSocketOptions.StartTls"/>
+		/// and the IMAP server does not support the STARTTLS extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.Net.Sockets.SocketException">
+		/// A socket error occurred trying to connect to the remote host.
+		/// </exception>
+		/// <exception cref="SslHandshakeException">
+		/// An error occurred during the SSL/TLS negotiations.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// An IMAP command failed.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public override Task ConnectAsync (string host, int port = 0, SecureSocketOptions options = SecureSocketOptions.Auto, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return ConnectAsync (host, port, options, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously establish a connection to the specified IMAP or IMAP/S server using the provided socket.
+		/// </summary>
+		/// <remarks>
+		/// <para>Establishes a connection to the specified IMAP or IMAP/S server using
+		/// the provided socket.</para>
+		/// <para>If the <paramref name="options"/> has a value of
+		/// <see cref="SecureSocketOptions.Auto"/>, then the <paramref name="port"/> is used
+		/// to determine the default security options. If the <paramref name="port"/> has a value
+		/// of <c>993</c>, then the default options used will be
+		/// <see cref="SecureSocketOptions.SslOnConnect"/>. All other values will use
+		/// <see cref="SecureSocketOptions.StartTlsWhenAvailable"/>.</para>
+		/// <para>Once a connection is established, properties such as
+		/// <see cref="AuthenticationMechanisms"/> and <see cref="Capabilities"/> will be
+		/// populated.</para>
+		/// <note type="info">With the exception of using the <paramref name="port"/> to determine the
+		/// default <see cref="SecureSocketOptions"/> to use when the <paramref name="options"/> value
+		/// is <see cref="SecureSocketOptions.Auto"/>, the <paramref name="host"/> and
+		/// <paramref name="port"/> parameters are only used for logging purposes.</note>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="socket">The socket to use for the connection.</param>
+		/// <param name="host">The host name to connect to.</param>
+		/// <param name="port">The port to connect to. If the specified port is <c>0</c>, then the default port will be used.</param>
+		/// <param name="options">The secure socket options to when connecting.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="socket"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="host"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="port"/> is not between <c>0</c> and <c>65535</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <para><paramref name="socket"/> is not connected.</para>
+		/// <para>-or-</para>
+		/// The <paramref name="host"/> is a zero-length string.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// The <see cref="ImapClient"/> is already connected.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <paramref name="options"/> was set to
+		/// <see cref="MailKit.Security.SecureSocketOptions.StartTls"/>
+		/// and the IMAP server does not support the STARTTLS extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="SslHandshakeException">
+		/// An error occurred during the SSL/TLS negotiations.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// An IMAP command failed.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public override Task ConnectAsync (Socket socket, string host, int port = 0, SecureSocketOptions options = SecureSocketOptions.Auto, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return ConnectAsync (socket, host, port, options, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously establish a connection to the specified IMAP or IMAP/S server using the provided stream.
+		/// </summary>
+		/// <remarks>
+		/// <para>Establishes a connection to the specified IMAP or IMAP/S server using
+		/// the provided stream.</para>
+		/// <para>If the <paramref name="options"/> has a value of
+		/// <see cref="SecureSocketOptions.Auto"/>, then the <paramref name="port"/> is used
+		/// to determine the default security options. If the <paramref name="port"/> has a value
+		/// of <c>993</c>, then the default options used will be
+		/// <see cref="SecureSocketOptions.SslOnConnect"/>. All other values will use
+		/// <see cref="SecureSocketOptions.StartTlsWhenAvailable"/>.</para>
+		/// <para>Once a connection is established, properties such as
+		/// <see cref="AuthenticationMechanisms"/> and <see cref="Capabilities"/> will be
+		/// populated.</para>
+		/// <note type="info">With the exception of using the <paramref name="port"/> to determine the
+		/// default <see cref="SecureSocketOptions"/> to use when the <paramref name="options"/> value
+		/// is <see cref="SecureSocketOptions.Auto"/>, the <paramref name="host"/> and
+		/// <paramref name="port"/> parameters are only used for logging purposes.</note>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="stream">The stream to use for the connection.</param>
+		/// <param name="host">The host name to connect to.</param>
+		/// <param name="port">The port to connect to. If the specified port is <c>0</c>, then the default port will be used.</param>
+		/// <param name="options">The secure socket options to when connecting.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="stream"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="host"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="port"/> is not between <c>0</c> and <c>65535</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// The <paramref name="host"/> is a zero-length string.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// The <see cref="ImapClient"/> is already connected.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <paramref name="options"/> was set to
+		/// <see cref="MailKit.Security.SecureSocketOptions.StartTls"/>
+		/// and the IMAP server does not support the STARTTLS extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="SslHandshakeException">
+		/// An error occurred during the SSL/TLS negotiations.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// An IMAP command failed.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public override Task ConnectAsync (Stream stream, string host, int port = 0, SecureSocketOptions options = SecureSocketOptions.Auto, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return ConnectAsync (stream, host, port, options, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously disconnect the service.
+		/// </summary>
+		/// <remarks>
+		/// If <paramref name="quit"/> is <c>true</c>, a <c>LOGOUT</c> command will be issued in order to disconnect cleanly.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadMessages"/>
+		/// </example>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="quit">If set to <c>true</c>, a <c>LOGOUT</c> command will be issued in order to disconnect cleanly.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		public override Task DisconnectAsync (bool quit, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return DisconnectAsync (quit, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously ping the IMAP server to keep the connection alive.
+		/// </summary>
+		/// <remarks>
+		/// <para>The <c>NOOP</c> command is typically used to keep the connection with the IMAP server
+		/// alive. When a client goes too long (typically 30 minutes) without sending any commands to the
+		/// IMAP server, the IMAP server will close the connection with the client, forcing the client to
+		/// reconnect before it can send any more commands.</para>
+		/// <para>The <c>NOOP</c> command also provides a great way for a client to check for new
+		/// messages.</para>
+		/// <para>When the IMAP server receives a <c>NOOP</c> command, it will reply to the client with a
+		/// list of pending updates such as <c>EXISTS</c> and <c>RECENT</c> counts on the currently
+		/// selected folder. To receive these notifications, subscribe to the
+		/// <see cref="MailFolder.CountChanged"/> and <see cref="MailFolder.RecentChanged"/> events,
+		/// respectively.</para>
+		/// <para>For more information about the <c>NOOP</c> command, see
+		/// <a href="https://tools.ietf.org/html/rfc3501#section-6.1.2">rfc3501</a>.</para>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapIdleExample.cs"/>
+		/// </example>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the NOOP command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		public override Task NoOpAsync (CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return NoOpAsync (true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously toggle the <see cref="ImapClient"/> into the IDLE state.
+		/// </summary>
+		/// <remarks>
+		/// <para>When a client enters the IDLE state, the IMAP server will send
+		/// events to the client as they occur on the selected folder. These events
+		/// may include notifications of new messages arriving, expunge notifications,
+		/// flag changes, etc.</para>
+		/// <para>Due to the nature of the IDLE command, a folder must be selected
+		/// before a client can enter into the IDLE state. This can be done by
+		/// opening a folder using
+		/// <see cref="MailKit.MailFolder.Open(FolderAccess,System.Threading.CancellationToken)"/>
+		/// or any of the other variants.</para>
+		/// <para>While the IDLE command is running, no other commands may be issued until the
+		/// <paramref name="doneToken"/> is cancelled.</para>
+		/// <note type="note">It is especially important to cancel the <paramref name="doneToken"/>
+		/// before cancelling the <paramref name="cancellationToken"/> when using SSL or TLS due to
+		/// the fact that <see cref="System.Net.Security.SslStream"/> cannot be polled.</note>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="doneToken">The cancellation token used to return to the non-idle state.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="doneToken"/> must be cancellable (i.e. <see cref="System.Threading.CancellationToken.None"/> cannot be used).
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// A <see cref="ImapFolder"/> has not been opened.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the IDLE extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the IDLE command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		public Task IdleAsync (CancellationToken doneToken, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return IdleAsync (doneToken, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously request the specified notification events from the IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// <para>The <a href="https://tools.ietf.org/html/rfc5465">NOTIFY</a> command is used to expand
+		/// which notifications the client wishes to be notified about, including status notifications
+		/// about folders other than the currently selected folder. It can also be used to automatically
+		/// FETCH information about new messages that have arrived in the currently selected folder.</para>
+		/// <para>This, combined with <see cref="IdleAsync(CancellationToken, CancellationToken)"/>,
+		/// can be used to get instant notifications for changes to any of the specified folders.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="status"><c>true</c> if the server should immediately notify the client of the
+		/// selected folder's status; otherwise, <c>false</c>.</param>
+		/// <param name="eventGroups">The specific event groups that the client would like to receive notifications for.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="eventGroups"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="eventGroups"/> is empty.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// One or more <see cref="ImapEventGroup"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the NOTIFY extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the NOTIFY command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		public Task NotifyAsync (bool status, IList<ImapEventGroup> eventGroups, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return NotifyAsync (status, eventGroups, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously disable any previously requested notification events from the IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// Disables any notification events requested in a prior call to 
+		/// <see cref="NotifyAsync(bool, IList{ImapEventGroup}, CancellationToken)"/>.
+		/// request.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the NOTIFY extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the NOTIFY command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		public Task DisableNotifyAsync (CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return DisableNotifyAsync (true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously get all of the folders within the specified namespace.
+		/// </summary>
+		/// <remarks>
+		/// Gets all of the folders within the specified namespace.
+		/// </remarks>
+		/// <returns>The folders.</returns>
+		/// <param name="namespace">The namespace.</param>
+		/// <param name="items">The status items to pre-populate.</param>
+		/// <param name="subscribedOnly">If set to <c>true</c>, only subscribed folders will be listed.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="namespace"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The namespace folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the LIST or LSUB command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		public override Task<IList<IMailFolder>> GetFoldersAsync (FolderNamespace @namespace, StatusItems items = StatusItems.None, bool subscribedOnly = false, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return GetFoldersAsync (@namespace, items, subscribedOnly, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously get the folder for the specified path.
+		/// </summary>
+		/// <remarks>
+		/// Gets the folder for the specified path.
+		/// </remarks>
+		/// <returns>The folder.</returns>
+		/// <param name="path">The folder path.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="path"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="FolderNotFoundException">
+		/// The folder could not be found.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the IDLE command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		public override async Task<IMailFolder> GetFolderAsync (string path, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			if (path == null)
+				throw new ArgumentNullException (nameof (path));
+
+			CheckDisposed ();
+			CheckConnected ();
+			CheckAuthenticated ();
+
+			return await engine.GetFolderAsync (path, true, cancellationToken).ConfigureAwait (false);
+		}
+
+		/// <summary>
+		/// Asynchronously gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata value.</returns>
+		/// <param name="tag">The metadata tag.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the METADATA or METADATA-SERVER extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override Task<string> GetMetadataAsync (MetadataTag tag, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return GetMetadataAsync (tag, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified metadata.
+		/// </remarks>
+		/// <returns>The requested metadata.</returns>
+		/// <param name="options">The metadata options.</param>
+		/// <param name="tags">The metadata tags.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="options"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="tags"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the METADATA or METADATA-SERVER extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override Task<MetadataCollection> GetMetadataAsync (MetadataOptions options, IEnumerable<MetadataTag> tags, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return GetMetadataAsync (options, tags, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously gets the specified metadata.
+		/// </summary>
+		/// <remarks>
+		/// Sets the specified metadata.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="metadata">The metadata.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="metadata"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the METADATA or METADATA-SERVER extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override Task SetMetadataAsync (MetadataCollection metadata, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return SetMetadataAsync (metadata, true, cancellationToken);
+		}
+	}
+}

src/MailKit/Net/Imap/IImapClient.cs

@@ -0,0 +1,628 @@
+//
+// IImapClient.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Threading;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// An interface for an IMAP client.
+	/// </summary>
+	/// <remarks>
+	/// Implemented by <see cref="MailKit.Net.Imap.ImapClient"/>.
+	/// </remarks>
+	public interface IImapClient : IMailStore
+	{
+		/// <summary>
+		/// Get the capabilities supported by the IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// The capabilities will not be known until a successful connection has been made via one of
+		/// the <a href="Overload_MailKit_Net_Imap_ImapClient_Connect.htm">Connect</a> methods and may
+		/// change as a side-effect of calling one of the
+		/// <a href="Overload_MailKit_Net_Imap_ImapClient_Authenticate.htm">Authenticate</a>
+		/// methods.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The capabilities.</value>
+		/// <exception cref="System.ArgumentException">
+		/// Capabilities cannot be enabled, they may only be disabled.
+		/// </exception>
+		ImapCapabilities Capabilities { get; set; }
+
+		/// <summary>
+		/// Gets the maximum size of a message that can be appended to a folder.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the maximum size of a message, in bytes, that can be appended to a folder.</para>
+		/// <note type="note">If the value is not set, then the limit is unspecified.</note>
+		/// </remarks>
+		/// <value>The append limit.</value>
+		uint? AppendLimit { get; }
+
+		/// <summary>
+		/// Gets the internationalization level supported by the IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the internationalization level supported by the IMAP server.</para>
+		/// <para>For more information, see
+		/// <a href="https://tools.ietf.org/html/rfc5255#section-4">section 4 of rfc5255</a>.</para>
+		/// </remarks>
+		/// <value>The internationalization level.</value>
+		int InternationalizationLevel { get; }
+
+		/// <summary>
+		/// Get the access rights supported by the IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// These rights are additional rights supported by the IMAP server beyond the standard rights
+		/// defined in <a href="https://tools.ietf.org/html/rfc4314#section-2.1">section 2.1 of rfc4314</a>
+		/// and will not be populated until the client is successfully connected.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The rights.</value>
+		AccessRights Rights { get; }
+
+		/// <summary>
+		/// Get whether or not the client is currently in the IDLE state.
+		/// </summary>
+		/// <remarks>
+		/// Gets whether or not the client is currently in the IDLE state.
+		/// </remarks>
+		/// <value><c>true</c> if an IDLE command is active; otherwise, <c>false</c>.</value>
+		bool IsIdle { get; }
+
+		/// <summary>
+		/// Enable compression over the IMAP connection.
+		/// </summary>
+		/// <remarks>
+		/// <para>Enables compression over the IMAP connection.</para>
+		/// <para>If the IMAP server supports the <see cref="ImapCapabilities.Compress"/> extension,
+		/// it is possible at any point after connecting to enable compression to reduce network
+		/// bandwidth usage. Ideally, this method should be called before authenticating.</para>
+		/// </remarks>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Compression must be enabled before a folder has been selected.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the <see cref="ImapCapabilities.Compress"/> extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the COMPRESS command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		void Compress (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously enable compression over the IMAP connection.
+		/// </summary>
+		/// <remarks>
+		/// <para>Asynchronously enables compression over the IMAP connection.</para>
+		/// <para>If the IMAP server supports the <see cref="ImapCapabilities.Compress"/> extension,
+		/// it is possible at any point after connecting to enable compression to reduce network
+		/// bandwidth usage. Ideally, this method should be called before authenticating.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Compression must be enabled before a folder has been selected.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the COMPRESS extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the COMPRESS command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		Task CompressAsync (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Enable the UTF8=ACCEPT extension.
+		/// </summary>
+		/// <remarks>
+		/// Enables the <a href="https://tools.ietf.org/html/rfc6855">UTF8=ACCEPT</a> extension.
+		/// </remarks>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// UTF8=ACCEPT needs to be enabled before selecting a folder.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the UTF8=ACCEPT extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the ENABLE command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		void EnableUTF8 (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously enable the UTF8=ACCEPT extension.
+		/// </summary>
+		/// <remarks>
+		/// Enables the <a href="https://tools.ietf.org/html/rfc6855">UTF8=ACCEPT</a> extension.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// UTF8=ACCEPT needs to be enabled before selecting a folder.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the UTF8=ACCEPT extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the ENABLE command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		Task EnableUTF8Async (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Identify the client implementation to the server and obtain the server implementation details.
+		/// </summary>
+		/// <remarks>
+		/// <para>Passes along the client implementation details to the server while also obtaining implementation
+		/// details from the server.</para>
+		/// <para>If the <paramref name="clientImplementation"/> is <c>null</c> or no properties have been set, no
+		/// identifying information will be sent to the server.</para>
+		/// <note type="security">
+		/// <para>Security Implications</para>
+		/// <para>This command has the danger of violating the privacy of users if misused. Clients should
+		/// notify users that they send the ID command.</para>
+		/// <para>It is highly desirable that implementations provide a method of disabling ID support, perhaps by
+		/// not calling this method at all, or by passing <c>null</c> as the <paramref name="clientImplementation"/>
+		/// argument.</para>
+		/// <para>Implementors must exercise extreme care in adding properties to the <paramref name="clientImplementation"/>.
+		/// Some properties, such as a processor ID number, Ethernet address, or other unique (or mostly unique) identifier
+		/// would allow tracking of users in ways that violate user privacy expectations and may also make it easier for
+		/// attackers to exploit security holes in the client.</para>
+		/// </note>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <returns>The implementation details of the server if available; otherwise, <c>null</c>.</returns>
+		/// <param name="clientImplementation">The client implementation.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the ID extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the ID command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		ImapImplementation Identify (ImapImplementation clientImplementation, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously identify the client implementation to the server and obtain the server implementation details.
+		/// </summary>
+		/// <remarks>
+		/// <para>Passes along the client implementation details to the server while also obtaining implementation
+		/// details from the server.</para>
+		/// <para>If the <paramref name="clientImplementation"/> is <c>null</c> or no properties have been set, no
+		/// identifying information will be sent to the server.</para>
+		/// <note type="security">
+		/// <para>Security Implications</para>
+		/// <para>This command has the danger of violating the privacy of users if misused. Clients should
+		/// notify users that they send the ID command.</para>
+		/// <para>It is highly desirable that implementations provide a method of disabling ID support, perhaps by
+		/// not calling this method at all, or by passing <c>null</c> as the <paramref name="clientImplementation"/>
+		/// argument.</para>
+		/// <para>Implementors must exercise extreme care in adding properties to the <paramref name="clientImplementation"/>.
+		/// Some properties, such as a processor ID number, Ethernet address, or other unique (or mostly unique) identifier
+		/// would allow tracking of users in ways that violate user privacy expectations and may also make it easier for
+		/// attackers to exploit security holes in the client.</para>
+		/// </note>
+		/// </remarks>
+		/// <returns>The implementation details of the server if available; otherwise, <c>null</c>.</returns>
+		/// <param name="clientImplementation">The client implementation.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the ID extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the ID command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		Task<ImapImplementation> IdentifyAsync (ImapImplementation clientImplementation, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Toggle the <see cref="ImapClient"/> into the IDLE state.
+		/// </summary>
+		/// <remarks>
+		/// <para>When a client enters the IDLE state, the IMAP server will send
+		/// events to the client as they occur on the selected folder. These events
+		/// may include notifications of new messages arriving, expunge notifications,
+		/// flag changes, etc.</para>
+		/// <para>Due to the nature of the IDLE command, a folder must be selected
+		/// before a client can enter into the IDLE state. This can be done by
+		/// opening a folder using
+		/// <see cref="MailKit.MailFolder.Open(FolderAccess,System.Threading.CancellationToken)"/>
+		/// or any of the other variants.</para>
+		/// <para>While the IDLE command is running, no other commands may be issued until the
+		/// <paramref name="doneToken"/> is cancelled.</para>
+		/// <note type="note">It is especially important to cancel the <paramref name="doneToken"/>
+		/// before cancelling the <paramref name="cancellationToken"/> when using SSL or TLS due to
+		/// the fact that <see cref="System.Net.Security.SslStream"/> cannot be polled.</note>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapIdleExample.cs"/>
+		/// </example>
+		/// <param name="doneToken">The cancellation token used to return to the non-idle state.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="doneToken"/> must be cancellable (i.e. <see cref="System.Threading.CancellationToken.None"/> cannot be used).
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// A <see cref="ImapFolder"/> has not been opened.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the IDLE extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the IDLE command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		void Idle (CancellationToken doneToken, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously toggle the <see cref="ImapClient"/> into the IDLE state.
+		/// </summary>
+		/// <remarks>
+		/// <para>When a client enters the IDLE state, the IMAP server will send
+		/// events to the client as they occur on the selected folder. These events
+		/// may include notifications of new messages arriving, expunge notifications,
+		/// flag changes, etc.</para>
+		/// <para>Due to the nature of the IDLE command, a folder must be selected
+		/// before a client can enter into the IDLE state. This can be done by
+		/// opening a folder using
+		/// <see cref="MailKit.MailFolder.Open(FolderAccess,System.Threading.CancellationToken)"/>
+		/// or any of the other variants.</para>
+		/// <para>While the IDLE command is running, no other commands may be issued until the
+		/// <paramref name="doneToken"/> is cancelled.</para>
+		/// <note type="note">It is especially important to cancel the <paramref name="doneToken"/>
+		/// before cancelling the <paramref name="cancellationToken"/> when using SSL or TLS due to
+		/// the fact that <see cref="System.Net.Security.SslStream"/> cannot be polled.</note>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="doneToken">The cancellation token used to return to the non-idle state.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="doneToken"/> must be cancellable (i.e. <see cref="System.Threading.CancellationToken.None"/> cannot be used).
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// A <see cref="ImapFolder"/> has not been opened.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the IDLE extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the IDLE command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		Task IdleAsync (CancellationToken doneToken, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Request the specified notification events from the IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// <para>The <a href="https://tools.ietf.org/html/rfc5465">NOTIFY</a> command is used to expand
+		/// which notifications the client wishes to be notified about, including status notifications
+		/// about folders other than the currently selected folder. It can also be used to automatically
+		/// FETCH information about new messages that have arrived in the currently selected folder.</para>
+		/// <para>This, combined with <see cref="Idle(CancellationToken, CancellationToken)"/>,
+		/// can be used to get instant notifications for changes to any of the specified folders.</para>
+		/// </remarks>
+		/// <param name="status"><c>true</c> if the server should immediately notify the client of the
+		/// selected folder's status; otherwise, <c>false</c>.</param>
+		/// <param name="eventGroups">The specific event groups that the client would like to receive notifications for.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="eventGroups"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="eventGroups"/> is empty.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// One or more <see cref="ImapEventGroup"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the NOTIFY extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the NOTIFY command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		void Notify (bool status, IList<ImapEventGroup> eventGroups, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously request the specified notification events from the IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// <para>The <a href="https://tools.ietf.org/html/rfc5465">NOTIFY</a> command is used to expand
+		/// which notifications the client wishes to be notified about, including status notifications
+		/// about folders other than the currently selected folder. It can also be used to automatically
+		/// FETCH information about new messages that have arrived in the currently selected folder.</para>
+		/// <para>This, combined with <see cref="IdleAsync(CancellationToken, CancellationToken)"/>,
+		/// can be used to get instant notifications for changes to any of the specified folders.</para>
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="status"><c>true</c> if the server should immediately notify the client of the
+		/// selected folder's status; otherwise, <c>false</c>.</param>
+		/// <param name="eventGroups">The specific event groups that the client would like to receive notifications for.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="eventGroups"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="eventGroups"/> is empty.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// One or more <see cref="ImapEventGroup"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the NOTIFY extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the NOTIFY command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		Task NotifyAsync (bool status, IList<ImapEventGroup> eventGroups, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Disable any previously requested notification events from the IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// Disables any notification events requested in a prior call to 
+		/// <see cref="Notify(bool, IList{ImapEventGroup}, CancellationToken)"/>.
+		/// request.
+		/// </remarks>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the NOTIFY extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the NOTIFY command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		void DisableNotify (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously disable any previously requested notification events from the IMAP server.
+		/// </summary>
+		/// <remarks>
+		/// Disables any notification events requested in a prior call to 
+		/// <see cref="NotifyAsync(bool, IList{ImapEventGroup}, CancellationToken)"/>.
+		/// request.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the NOTIFY extension.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied to the NOTIFY command with a NO or BAD response.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server responded with an unexpected token.
+		/// </exception>
+		Task DisableNotifyAsync (CancellationToken cancellationToken = default (CancellationToken));
+	}
+}

src/MailKit/Net/Imap/IImapFolder.cs

@@ -0,0 +1,869 @@
+//
+// IImapFolder.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Threading;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+using MimeKit;
+
+using MailKit.Search;
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// An interface for an IMAP folder.
+	/// </summary>
+	/// <remarks>
+	/// Implemented by <see cref="MailKit.Net.Imap.ImapFolder"/>.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadMessages"/>
+	/// </example>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="DownloadBodyParts"/>
+	/// </example>
+	public interface IImapFolder : IMailFolder
+	{
+		/// <summary>
+		/// Get the specified body part headers.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified body part headers.
+		/// </remarks>
+		/// <returns>The body part headers.</returns>
+		/// <param name="uid">The UID of the message.</param>
+		/// <param name="partSpecifier">The body part specifier.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="partSpecifier"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="uid"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="MessageNotFoundException">
+		/// The IMAP server did not return the requested body part headers.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		HeaderList GetHeaders (UniqueId uid, string partSpecifier, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the specified body part headers.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified body part headers.
+		/// </remarks>
+		/// <returns>The body part headers.</returns>
+		/// <param name="uid">The UID of the message.</param>
+		/// <param name="partSpecifier">The body part specifier.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="partSpecifier"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="uid"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="MessageNotFoundException">
+		/// The IMAP server did not return the requested body part headers.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		Task<HeaderList> GetHeadersAsync (UniqueId uid, string partSpecifier, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the specified body part headers.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified body part headers.
+		/// </remarks>
+		/// <returns>The body part headers.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="partSpecifier">The body part specifier.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="partSpecifier"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="MessageNotFoundException">
+		/// The IMAP server did not return the requested body part headers.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		HeaderList GetHeaders (int index, string partSpecifier, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the specified body part headers.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified body part headers.
+		/// </remarks>
+		/// <returns>The body part headers.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="partSpecifier">The body part specifier.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="partSpecifier"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="MessageNotFoundException">
+		/// The IMAP server did not return the requested body part headers.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		Task<HeaderList> GetHeadersAsync (int index, string partSpecifier, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the specified body part.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified body part.
+		/// </remarks>
+		/// <returns>The body part.</returns>
+		/// <param name="uid">The UID of the message.</param>
+		/// <param name="partSpecifier">The body part specifier.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="partSpecifier"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="uid"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="MessageNotFoundException">
+		/// The IMAP server did not return the requested message body.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		MimeEntity GetBodyPart (UniqueId uid, string partSpecifier, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the specified body part.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified body part.
+		/// </remarks>
+		/// <returns>The body part.</returns>
+		/// <param name="uid">The UID of the message.</param>
+		/// <param name="partSpecifier">The body part specifier.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="partSpecifier"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="uid"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="MessageNotFoundException">
+		/// The IMAP server did not return the requested message body.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		Task<MimeEntity> GetBodyPartAsync (UniqueId uid, string partSpecifier, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the specified body part.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified body part.
+		/// </remarks>
+		/// <returns>The body part.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="partSpecifier">The body part specifier.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="partSpecifier"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="MessageNotFoundException">
+		/// The IMAP server did not return the requested message.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		MimeEntity GetBodyPart (int index, string partSpecifier, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the specified body part.
+		/// </summary>
+		/// <remarks>
+		/// Gets the specified body part.
+		/// </remarks>
+		/// <returns>The body part.</returns>
+		/// <param name="index">The index of the message.</param>
+		/// <param name="partSpecifier">The body part specifier.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="partSpecifier"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="index"/> is out of range.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="MessageNotFoundException">
+		/// The IMAP server did not return the requested message.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		Task<MimeEntity> GetBodyPartAsync (int index, string partSpecifier, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the streams for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the streams for the specified messages.</para>
+		/// </remarks>
+		/// <param name="uids">The uids of the messages.</param>
+		/// <param name="callback"></param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="uids"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="callback"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="uids"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		void GetStreams (IList<UniqueId> uids, ImapFetchStreamCallback callback, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the streams for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// <para>Asynchronously gets the streams for the specified messages.</para>
+		/// </remarks>
+		/// <returns>An awaitable task.</returns>
+		/// <param name="uids">The uids of the messages.</param>
+		/// <param name="callback"></param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="uids"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="callback"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="uids"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		Task GetStreamsAsync (IList<UniqueId> uids, ImapFetchStreamAsyncCallback callback, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the streams for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the streams for the specified messages.</para>
+		/// </remarks>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="callback"></param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="indexes"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="callback"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="indexes"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		void GetStreams (IList<int> indexes, ImapFetchStreamCallback callback, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the streams for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// <para>Asynchronously gets the streams for the specified messages.</para>
+		/// </remarks>
+		/// <returns>An awaitable task.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="callback"></param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="indexes"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="callback"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="indexes"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		Task GetStreamsAsync (IList<int> indexes, ImapFetchStreamAsyncCallback callback, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Get the streams for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// <para>Gets the streams for the specified messages.</para>
+		/// </remarks>
+		/// <param name="min">The minimum index.</param>
+		/// <param name="max">The maximum index, or <c>-1</c> to specify no upper bound.</param>
+		/// <param name="callback"></param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="min"/> is out of range.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="max"/> is out of range.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="callback"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		void GetStreams (int min, int max, ImapFetchStreamCallback callback, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Asynchronously get the streams for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// <para>Asynchronously gets the streams for the specified messages.</para>
+		/// </remarks>
+		/// <returns>An awaitable task.</returns>
+		/// <param name="min">The minimum index.</param>
+		/// <param name="max">The maximum index, or <c>-1</c> to specify no upper bound.</param>
+		/// <param name="callback"></param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="progress">The progress reporting mechanism.</param>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <para><paramref name="min"/> is out of range.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="max"/> is out of range.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="callback"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		Task GetStreamsAsync (int min, int max, ImapFetchStreamAsyncCallback callback, CancellationToken cancellationToken = default (CancellationToken), ITransferProgress progress = null);
+
+		/// <summary>
+		/// Search the folder for messages matching the specified query.
+		/// </summary>
+		/// <remarks>
+		/// Sends a <c>UID SEARCH</c> command with the specified query passed directly to the IMAP server
+		/// with no interpretation by MailKit. This means that the query may contain any arguments that a
+		/// <c>UID SEARCH</c> command is allowed to have according to the IMAP specifications and any
+		/// extensions that are supported, including <c>RETURN</c> parameters.
+		/// </remarks>
+		/// <returns>An array of matching UIDs.</returns>
+		/// <param name="query">The search query.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="query"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="query"/> is an empty string.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		SearchResults Search (string query, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously search the folder for messages matching the specified query.
+		/// </summary>
+		/// <remarks>
+		/// Sends a <c>UID SEARCH</c> command with the specified query passed directly to the IMAP server
+		/// with no interpretation by MailKit. This means that the query may contain any arguments that a
+		/// <c>UID SEARCH</c> command is allowed to have according to the IMAP specifications and any
+		/// extensions that are supported, including <c>RETURN</c> parameters.
+		/// </remarks>
+		/// <returns>An array of matching UIDs.</returns>
+		/// <param name="query">The search query.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="query"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="query"/> is an empty string.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		Task<SearchResults> SearchAsync (string query, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Sort messages matching the specified query.
+		/// </summary>
+		/// <remarks>
+		/// Sends a <c>UID SORT</c> command with the specified query passed directly to the IMAP server
+		/// with no interpretation by MailKit. This means that the query may contain any arguments that a
+		/// <c>UID SORT</c> command is allowed to have according to the IMAP specifications and any
+		/// extensions that are supported, including <c>RETURN</c> parameters.
+		/// </remarks>
+		/// <returns>An array of matching UIDs.</returns>
+		/// <param name="query">The search query.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="query"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="query"/> is an empty string.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the SORT extension.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		SearchResults Sort (string query, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously sort messages matching the specified query.
+		/// </summary>
+		/// <remarks>
+		/// Sends a <c>UID SORT</c> command with the specified query passed directly to the IMAP server
+		/// with no interpretation by MailKit. This means that the query may contain any arguments that a
+		/// <c>UID SORT</c> command is allowed to have according to the IMAP specifications and any
+		/// extensions that are supported, including <c>RETURN</c> parameters.
+		/// </remarks>
+		/// <returns>An array of matching UIDs.</returns>
+		/// <param name="query">The search query.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="query"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="query"/> is an empty string.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The IMAP server does not support the SORT extension.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		Task<SearchResults> SortAsync (string query, CancellationToken cancellationToken = default (CancellationToken));
+	}
+}

src/MailKit/Net/Imap/ImapCallbacks.cs

@@ -0,0 +1,68 @@
+//
+// ImapCallbacks.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MailKit.Net.Imap
+{
+	/// <summary>
+	/// A callback used when fetching message streams.
+	/// </summary>
+	/// <remarks>
+	/// <para>This callback is meant to be used with the various
+	/// <a href="Overload_MailKit_Net_Imap_ImapFolder_GetStreams.htm">GetStreams</a>
+	/// methods.</para>
+	/// <para>Once this callback returns, the stream argument will be disposed, so
+	/// it is important to consume the stream right away and not add it to a queue
+	/// for later processing.</para>
+	/// </remarks>
+	/// <param name="folder">The IMAP folder that the message belongs to.</param>
+	/// <param name="index">The index of the message in the folder.</param>
+	/// <param name="uid">The UID of the message in the folder.</param>
+	/// <param name="stream">The raw message (or part) stream.</param>
+	public delegate void ImapFetchStreamCallback (ImapFolder folder, int index, UniqueId uid, Stream stream);
+
+	/// <summary>
+	/// An asynchronous callback used when fetching message streams.
+	/// </summary>
+	/// <remarks>
+	/// <para>This callback is meant to be used with the various
+	/// <a href="Overload_MailKit_Net_Imap_ImapFolder_GetStreamsAsync.htm">GetStreamsAsync</a>
+	/// methods.</para>
+	/// <para>Once this callback returns, the stream argument will be disposed, so
+	/// it is important to consume the stream right away and not add it to a queue
+	/// for later processing.</para>
+	/// </remarks>
+	/// <returns>An awaitable task context.</returns>
+	/// <param name="folder">The IMAP folder that the message belongs to.</param>
+	/// <param name="index">The index of the message in the folder.</param>
+	/// <param name="uid">The UID of the message in the folder.</param>
+	/// <param name="stream">The raw message (or part) stream.</param>
+	/// <param name="cancellationToken">The cancellation token.</param>
+	public delegate Task ImapFetchStreamAsyncCallback (ImapFolder folder, int index, UniqueId uid, Stream stream, CancellationToken cancellationToken);
+}

src/MailKit/Net/Imap/ImapCapabilities.cs

@@ -0,0 +1,348 @@
+//
+// ImapCapabilities.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// Capabilities supported by an IMAP server.
+	/// </summary>
+	/// <remarks>
+	/// Capabilities are read as part of the response to the <c>CAPABILITY</c> command that
+	/// is issued during the connection and authentication phases of the
+	/// <see cref="ImapClient"/>.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+	/// </example>
+	[Flags]
+	public enum ImapCapabilities : ulong {
+		/// <summary>
+		/// The server does not support any additional extensions.
+		/// </summary>
+		None             = 0,
+
+		/// <summary>
+		/// The server implements the core IMAP4 commands.
+		/// </summary>
+		IMAP4            = 1L << 0,
+
+		/// <summary>
+		/// The server implements the core IMAP4rev1 commands.
+		/// </summary>
+		IMAP4rev1        = 1L << 1,
+
+		/// <summary>
+		/// The server implements the core IMAP4rev2 commands.
+		/// </summary>
+		IMAP4rev2        = 1L << 2,
+
+		/// <summary>
+		/// The server supports the <c>STATUS</c> command.
+		/// </summary>
+		Status           = 1L << 3,
+
+		/// <summary>
+		/// The server supports the ACL extension defined in <a href="https://tools.ietf.org/html/rfc2086">rfc2086</a>
+		/// and <a href="https://tools.ietf.org/html/rfc4314">rfc4314</a>.
+		/// </summary>
+		Acl              = 1L << 4,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2087">QUOTA</a> extension.
+		/// </summary>
+		Quota            = 1L << 5,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2088">LITERAL+</a> extension.
+		/// </summary>
+		LiteralPlus      = 1L << 6,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2177">IDLE</a> extension.
+		/// </summary>
+		Idle             = 1L << 7,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2193">MAILBOX-REFERRALS</a> extension.
+		/// </summary>
+		MailboxReferrals = 1L << 8,
+
+		/// <summary>
+		/// the server supports the <a href="https://tools.ietf.org/html/rfc2221">LOGIN-REFERRALS</a> extension.
+		/// </summary>
+		LoginReferrals   = 1L << 9,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2342">NAMESPACE</a> extension.
+		/// </summary>
+		Namespace        = 1L << 10,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2971">ID</a> extension.
+		/// </summary>
+		Id               = 1L << 11,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc3348">CHILDREN</a> extension.
+		/// </summary>
+		Children         = 1L << 12,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc3501">LOGINDISABLED</a> extension.
+		/// </summary>
+		LoginDisabled    = 1L << 13,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc3501">STARTTLS</a> extension.
+		/// </summary>
+		StartTLS         = 1L << 14,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc3502">MULTIAPPEND</a> extension.
+		/// </summary>
+		MultiAppend      = 1L << 15,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc3516">BINARY</a> content extension.
+		/// </summary>
+		Binary           = 1L << 16,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc3691">UNSELECT</a> extension.
+		/// </summary>
+		Unselect         = 1L << 17,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc4315">UIDPLUS</a> extension.
+		/// </summary>
+		UidPlus          = 1L << 18,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc4469">CATENATE</a> extension.
+		/// </summary>
+		Catenate         = 1L << 19,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc4551">CONDSTORE</a> extension.
+		/// </summary>
+		CondStore        = 1L << 20,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc4731">ESEARCH</a> extension.
+		/// </summary>
+		ESearch          = 1L << 21,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc4959">SASL-IR</a> extension.
+		/// </summary>
+		SaslIR           = 1L << 22,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc4978">COMPRESS</a> extension.
+		/// </summary>
+		Compress         = 1L << 23,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5032">WITHIN</a> extension.
+		/// </summary>
+		Within           = 1L << 24,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5161">ENABLE</a> extension.
+		/// </summary>
+		Enable           = 1L << 25,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5162">QRESYNC</a> extension.
+		/// </summary>
+		QuickResync      = 1L << 26,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5182">SEARCHRES</a> extension.
+		/// </summary>
+		SearchResults    = 1L << 27,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5256">SORT</a> extension.
+		/// </summary>
+		Sort             = 1L << 28,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5256">THREAD</a> extension.
+		/// </summary>
+		Thread           = 1L << 29,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5257">ANNOTATE</a> extension.
+		/// </summary>
+		Annotate         = 1L << 30,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5258">LIST-EXTENDED</a> extension.
+		/// </summary>
+		ListExtended     = 1L << 31,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5259">CONVERT</a> extension.
+		/// </summary>
+		Convert          = 1L << 32,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5255">LANGUAGE</a> extension.
+		/// </summary>
+		Language         = 1L << 33,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5255">I18NLEVEL</a> extension.
+		/// </summary>
+		I18NLevel        = 1L << 34,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5267">ESORT</a> extension.
+		/// </summary>
+		ESort            = 1L << 35,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5267">CONTEXT</a> extension.
+		/// </summary>
+		Context          = 1L << 36,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5464">METADATA</a> extension.
+		/// </summary>
+		Metadata         = 1L << 37,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5464">METADATA-SERVER</a> extension.
+		/// </summary>
+		MetadataServer   = 1L << 38,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5465">NOTIFY</a> extension.
+		/// </summary>
+		Notify           = 1L << 39,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5466">FILTERS</a> extension.
+		/// </summary>
+		Filters          = 1L << 40,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5819">LIST-STATUS</a> extension.
+		/// </summary>
+		ListStatus       = 1L << 41,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc5957">SORT=DISPLAY</a> extension.
+		/// </summary>
+		SortDisplay      = 1L << 42,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6154">CREATE-SPECIAL-USE</a> extension.
+		/// </summary>
+		CreateSpecialUse = 1L << 43,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6154">SPECIAL-USE</a> extension.
+		/// </summary>
+		SpecialUse       = 1L << 44,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6203">SEARCH=FUZZY</a> extension.
+		/// </summary>
+		FuzzySearch      = 1L << 45,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6237">MULTISEARCH</a> extension.
+		/// </summary>
+		MultiSearch      = 1L << 46,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6851">MOVE</a> extension.
+		/// </summary>
+		Move             = 1L << 47,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6855">UTF8=ACCEPT</a> extension.
+		/// </summary>
+		UTF8Accept       = 1L << 48,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6855">UTF8=ONLY</a> extension.
+		/// </summary>
+		UTF8Only         = 1L << 49,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc7888">LITERAL-</a> extension.
+		/// </summary>
+		LiteralMinus     = 1L << 50,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc7889">APPENDLIMIT</a> extension.
+		/// </summary>
+		AppendLimit      = 1L << 51,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc8437">UNAUTHENTICATE</a> extension.
+		/// </summary>
+		Unauthenticate   = 1L << 52,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc8438">STATUS=SIZE</a> extension.
+		/// </summary>
+		StatusSize       = 1L << 53,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc8440">LIST-MYRIGHTS</a> extension.
+		/// </summary>
+		ListMyRights     = 1L << 54,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc8474">OBJECTID</a> extension.
+		/// </summary>
+		ObjectID         = 1L << 55,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc8508">REPLACE</a> extension.
+		/// </summary>
+		Replace          = 1L << 56,
+
+		#region GMail Extensions
+
+		/// <summary>
+		/// The server supports the <a href="https://developers.google.com/gmail/imap_extensions">XLIST</a> extension (GMail).
+		/// </summary>
+		XList            = 1L << 60,
+
+		/// <summary>
+		/// The server supports the <a href="https://developers.google.com/gmail/imap_extensions">X-GM-EXT1</a> extension (GMail).
+		/// </summary>
+		GMailExt1        = 1L << 61
+
+		#endregion
+	}
+}

src/MailKit/Net/Imap/ImapCommand.cs

@@ -0,0 +1,918 @@
+//
+// ImapCommand.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.IO;
+using System.Text;
+using System.Threading;
+using System.Globalization;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+using MimeKit;
+using MimeKit.IO;
+using MimeKit.Utils;
+
+using SslStream = MailKit.Net.SslStream;
+using NetworkStream = MailKit.Net.NetworkStream;
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// An IMAP continuation handler.
+	/// </summary>
+	/// <remarks>
+	/// All exceptions thrown by the handler are considered fatal and will
+	/// force-disconnect the connection. If a non-fatal error occurs, set
+	/// it on the <see cref="ImapCommand.Exception"/> property.
+	/// </remarks>
+	delegate Task ImapContinuationHandler (ImapEngine engine, ImapCommand ic, string text, bool doAsync);
+
+	/// <summary>
+	/// An IMAP untagged response handler.
+	/// </summary>
+	/// <remarks>
+	/// <para>Most IMAP commands return their results in untagged responses.</para>
+	/// </remarks>
+	delegate Task ImapUntaggedHandler (ImapEngine engine, ImapCommand ic, int index, bool doAsync);
+
+	delegate void ImapCommandResetHandler (ImapCommand ic);
+
+	/// <summary>
+	/// IMAP command status.
+	/// </summary>
+	enum ImapCommandStatus {
+		Created,
+		Queued,
+		Active,
+		Complete,
+		Error
+	}
+
+	enum ImapLiteralType {
+		String,
+		//Stream,
+		MimeMessage
+	}
+
+	enum ImapStringType {
+		Atom,
+		QString,
+		Literal,
+		Nil
+	}
+
+	/// <summary>
+	/// An IMAP IDLE context.
+	/// </summary>
+	/// <remarks>
+	/// <para>An IMAP IDLE command does not work like normal commands. Unlike most commands,
+	/// the IDLE command does not end until the client sends a separate "DONE" command.</para>
+	/// <para>In order to facilitate this, the way this works is that the consumer of MailKit's
+	/// IMAP APIs provides a 'doneToken' which signals to the command-processing loop to
+	/// send the "DONE" command. Since, like every other IMAP command, it is also necessary to
+	/// provide a means of cancelling the IDLE command, it becomes necessary to link the
+	/// 'doneToken' and the 'cancellationToken' together.</para>
+	/// </remarks>
+	sealed class ImapIdleContext : IDisposable
+	{
+		static readonly byte[] DoneCommand = Encoding.ASCII.GetBytes ("DONE\r\n");
+		CancellationTokenRegistration registration;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapIdleContext"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.Net.Imap.ImapIdleContext"/>.
+		/// </remarks>
+		/// <param name="engine">The IMAP engine.</param>
+		/// <param name="doneToken">The done token.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		public ImapIdleContext (ImapEngine engine, CancellationToken doneToken, CancellationToken cancellationToken)
+		{
+			CancellationToken = cancellationToken;
+			DoneToken = doneToken;
+			Engine = engine;
+		}
+
+		/// <summary>
+		/// Get the engine.
+		/// </summary>
+		/// <remarks>
+		/// Gets the engine.
+		/// </remarks>
+		/// <value>The engine.</value>
+		public ImapEngine Engine {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the cancellation token.
+		/// </summary>
+		/// <remarks>
+		/// Get the cancellation token.
+		/// </remarks>
+		/// <value>The cancellation token.</value>
+		public CancellationToken CancellationToken {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the done token.
+		/// </summary>
+		/// <remarks>
+		/// Gets the done token.
+		/// </remarks>
+		/// <value>The done token.</value>
+		public CancellationToken DoneToken {
+			get; private set;
+		}
+
+#if false
+		/// <summary>
+		/// Get whether or not cancellation has been requested.
+		/// </summary>
+		/// <remarks>
+		/// Gets whether or not cancellation has been requested.
+		/// </remarks>
+		/// <value><c>true</c> if cancellation has been requested; otherwise, <c>false</c>.</value>
+		public bool IsCancellationRequested {
+			get { return CancellationToken.IsCancellationRequested; }
+		}
+
+		/// <summary>
+		/// Get whether or not the IDLE command should be ended.
+		/// </summary>
+		/// <remarks>
+		/// Gets whether or not the IDLE command should be ended.
+		/// </remarks>
+		/// <value><c>true</c> if the IDLE command should end; otherwise, <c>false</c>.</value>
+		public bool IsDoneRequested {
+			get { return DoneToken.IsCancellationRequested; }
+		}
+#endif
+
+		void IdleComplete ()
+		{
+			if (Engine.State == ImapEngineState.Idle) {
+				try {
+					Engine.Stream.Write (DoneCommand, 0, DoneCommand.Length, CancellationToken);
+					Engine.Stream.Flush (CancellationToken);
+				} catch {
+					return;
+				}
+
+				Engine.State = ImapEngineState.Selected;
+			}
+		}
+
+		/// <summary>
+		/// Callback method to be used as the ImapCommand's ContinuationHandler.
+		/// </summary>
+		/// <remarks>
+		/// Callback method to be used as the ImapCommand's ContinuationHandler.
+		/// </remarks>
+		/// <param name="engine">The ImapEngine.</param>
+		/// <param name="ic">The ImapCommand.</param>
+		/// <param name="text">The text.</param>
+		/// <param name="doAsync"><c>true</c> if the command is being run asynchronously; otherwise, <c>false</c>.</param>
+		/// <returns></returns>
+		public Task ContinuationHandler (ImapEngine engine, ImapCommand ic, string text, bool doAsync)
+		{
+			Engine.State = ImapEngineState.Idle;
+
+			registration = DoneToken.Register (IdleComplete);
+
+			return Task.FromResult (true);
+		}
+
+		/// <summary>
+		/// Releases all resource used by the <see cref="MailKit.Net.Imap.ImapIdleContext"/> object.
+		/// </summary>
+		/// <remarks>Call <see cref="Dispose"/> when you are finished using the <see cref="MailKit.Net.Imap.ImapIdleContext"/>. The
+		/// <see cref="Dispose"/> method leaves the <see cref="MailKit.Net.Imap.ImapIdleContext"/> in an unusable state. After
+		/// calling <see cref="Dispose"/>, you must release all references to the
+		/// <see cref="MailKit.Net.Imap.ImapIdleContext"/> so the garbage collector can reclaim the memory that the
+		/// <see cref="MailKit.Net.Imap.ImapIdleContext"/> was occupying.</remarks>
+		public void Dispose ()
+		{
+			registration.Dispose ();
+		}
+	}
+
+	/// <summary>
+	/// An IMAP literal object.
+	/// </summary>
+	/// <remarks>
+	/// The literal can be a string, byte[], Stream, or a MimeMessage.
+	/// </remarks>
+	class ImapLiteral
+	{
+		public readonly ImapLiteralType Type;
+		public readonly object Literal;
+		readonly FormatOptions format;
+		readonly Action<int> update;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapLiteral"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.Net.Imap.ImapLiteral"/>.
+		/// </remarks>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="message">The message.</param>
+		/// <param name="action">The progress update action.</param>
+		public ImapLiteral (FormatOptions options, MimeMessage message, Action<int> action = null)
+		{
+			format = options.Clone ();
+			format.NewLineFormat = NewLineFormat.Dos;
+
+			update = action;
+
+			Type = ImapLiteralType.MimeMessage;
+			Literal = message;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapLiteral"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.Net.Imap.ImapLiteral"/>.
+		/// </remarks>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="literal">The literal.</param>
+		public ImapLiteral (FormatOptions options, byte[] literal)
+		{
+			format = options.Clone ();
+			format.NewLineFormat = NewLineFormat.Dos;
+
+			Type = ImapLiteralType.String;
+			Literal = literal;
+		}
+
+		/// <summary>
+		/// Get the length of the literal, in bytes.
+		/// </summary>
+		/// <remarks>
+		/// Gets the length of the literal, in bytes.
+		/// </remarks>
+		/// <value>The length.</value>
+		public long Length {
+			get {
+				if (Type == ImapLiteralType.String)
+					return ((byte[]) Literal).Length;
+
+				using (var measure = new MeasuringStream ()) {
+					//if (Type == ImapLiteralType.Stream) {
+					//	var stream = (Stream) Literal;
+					//	stream.CopyTo (measure, 4096);
+					//	stream.Position = 0;
+
+					//	return measure.Length;
+					//}
+
+					((MimeMessage) Literal).WriteTo (format, measure);
+
+					return measure.Length;
+				}
+			}
+		}
+
+		/// <summary>
+		/// Write the literal to the specified stream.
+		/// </summary>
+		/// <remarks>
+		/// Writes the literal to the specified stream.
+		/// </remarks>
+		/// <param name="stream">The stream.</param>
+		/// <param name="doAsync">Whether the literal should be written asynchronously or not.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		public async Task WriteToAsync (ImapStream stream, bool doAsync, CancellationToken cancellationToken)
+		{
+			if (Type == ImapLiteralType.String) {
+				var bytes = (byte[]) Literal;
+
+				if (doAsync) {
+					await stream.WriteAsync (bytes, 0, bytes.Length, cancellationToken).ConfigureAwait (false);
+					await stream.FlushAsync (cancellationToken).ConfigureAwait (false);
+				} else {
+					stream.Write (bytes, 0, bytes.Length, cancellationToken);
+					stream.Flush (cancellationToken);
+				}
+				return;
+			}
+
+			//if (Type == ImapLiteralType.Stream) {
+			//	var literal = (Stream) Literal;
+			//	var buf = new byte[4096];
+			//	int nread;
+
+			//	if (doAsync) {
+			//		while ((nread = await literal.ReadAsync (buf, 0, buf.Length, cancellationToken).ConfigureAwait (false)) > 0)
+			//			await stream.WriteAsync (buf, 0, nread, cancellationToken).ConfigureAwait (false);
+
+			//		await stream.FlushAsync (cancellationToken).ConfigureAwait (false);
+			//	} else {
+			//		while ((nread = literal.Read (buf, 0, buf.Length)) > 0)
+			//			stream.Write (buf, 0, nread, cancellationToken);
+
+			//		stream.Flush (cancellationToken);
+			//	}
+			//	return;
+			//}
+
+			var message = (MimeMessage) Literal;
+
+			using (var s = new ProgressStream (stream, update)) {
+				if (doAsync) {
+					await message.WriteToAsync (format, s, cancellationToken).ConfigureAwait (false);
+					await s.FlushAsync (cancellationToken).ConfigureAwait (false);
+				} else {
+					message.WriteTo (format, s, cancellationToken);
+					s.Flush (cancellationToken);
+				}
+			}
+		}
+	}
+
+	/// <summary>
+	/// A partial IMAP command.
+	/// </summary>
+	/// <remarks>
+	/// IMAP commands that contain literal strings are broken up into multiple parts
+	/// in case the IMAP server does not support the LITERAL+ extension. These parts
+	/// are then sent individually as we receive "+" responses from the server.
+	/// </remarks>
+	class ImapCommandPart
+	{
+		public readonly byte[] Command;
+		public readonly ImapLiteral Literal;
+		public readonly bool WaitForContinuation;
+
+		public ImapCommandPart (byte[] command, ImapLiteral literal, bool wait = true)
+		{
+			WaitForContinuation = wait;
+			Command = command;
+			Literal = literal;
+		}
+	}
+
+	/// <summary>
+	/// An IMAP command.
+	/// </summary>
+	class ImapCommand
+	{
+		static readonly byte[] UTF8LiteralTokenPrefix = Encoding.ASCII.GetBytes ("UTF8 (~{");
+		static readonly byte[] LiteralTokenSuffix = { (byte) '}', (byte) '\r', (byte) '\n' };
+		static readonly byte[] Nil = { (byte) 'N', (byte) 'I', (byte) 'L' };
+		static readonly byte[] NewLine = { (byte) '\r', (byte) '\n' };
+		static readonly byte[] LiteralTokenPrefix = { (byte) '{' };
+
+		public Dictionary<string, ImapUntaggedHandler> UntaggedHandlers { get; private set; }
+		public ImapContinuationHandler ContinuationHandler { get; set; }
+		public CancellationToken CancellationToken { get; private set; }
+		public ImapCommandStatus Status { get; internal set; }
+		public ImapCommandResponse Response { get; internal set; }
+		public ITransferProgress Progress { get; internal set; }
+		public Exception Exception { get; internal set; }
+		public readonly List<ImapResponseCode> RespCodes;
+		public string ResponseText { get; internal set; }
+		public ImapFolder Folder { get; private set; }
+		public object UserData { get; internal set; }
+		public bool ListReturnsSubscribed { get; internal set; }
+		public bool Logout { get; private set; }
+		public bool Lsub { get; internal set; }
+		public string Tag { get; private set; }
+		public bool Bye { get; internal set; }
+
+		readonly List<ImapCommandPart> parts = new List<ImapCommandPart> ();
+		readonly ImapEngine Engine;
+		long totalSize, nwritten;
+		int current;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapCommand"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.Net.Imap.ImapCommand"/>.
+		/// </remarks>
+		/// <param name="engine">The IMAP engine that will be sending the command.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="folder">The IMAP folder that the command operates on.</param>
+		/// <param name="options">The formatting options.</param>
+		/// <param name="format">The command format.</param>
+		/// <param name="args">The command arguments.</param>
+		public ImapCommand (ImapEngine engine, CancellationToken cancellationToken, ImapFolder folder, FormatOptions options, string format, params object[] args)
+		{
+			UntaggedHandlers = new Dictionary<string, ImapUntaggedHandler> (StringComparer.OrdinalIgnoreCase);
+			Logout = format.Equals ("LOGOUT\r\n", StringComparison.Ordinal);
+			RespCodes = new List<ImapResponseCode> ();
+			CancellationToken = cancellationToken;
+			Response = ImapCommandResponse.None;
+			Status = ImapCommandStatus.Created;
+			Engine = engine;
+			Folder = folder;
+
+			using (var builder = new MemoryStream ()) {
+				byte[] buf, utf8 = new byte[8];
+				int argc = 0;
+				string str;
+
+				for (int i = 0; i < format.Length; i++) {
+					if (format[i] == '%') {
+						switch (format[++i]) {
+						case '%': // a literal %
+							builder.WriteByte ((byte) '%');
+							break;
+						case 'd': // an integer
+							str = ((int) args[argc++]).ToString (CultureInfo.InvariantCulture);
+							buf = Encoding.ASCII.GetBytes (str);
+							builder.Write (buf, 0, buf.Length);
+							break;
+						case 'u': // an unsigned integer
+							str = ((uint) args[argc++]).ToString (CultureInfo.InvariantCulture);
+							buf = Encoding.ASCII.GetBytes (str);
+							builder.Write (buf, 0, buf.Length);
+							break;
+						case 's':
+							str = (string) args[argc++];
+							buf = Encoding.ASCII.GetBytes (str);
+							builder.Write (buf, 0, buf.Length);
+							break;
+						case 'F': // an ImapFolder
+							var utf7 = ((ImapFolder) args[argc++]).EncodedName;
+							AppendString (options, true, builder, utf7);
+							break;
+						case 'L': // a MimeMessage or a byte[]
+							var arg = args[argc++];
+							ImapLiteral literal;
+							byte[] prefix;
+
+							if (arg is MimeMessage message) {
+								prefix = options.International ? UTF8LiteralTokenPrefix : LiteralTokenPrefix;
+								literal = new ImapLiteral (options, message, UpdateProgress);
+							} else {
+								literal = new ImapLiteral (options, (byte[]) arg);
+								prefix = LiteralTokenPrefix;
+							}
+
+							var length = literal.Length;
+							bool wait = true;
+
+							builder.Write (prefix, 0, prefix.Length);
+							buf = Encoding.ASCII.GetBytes (length.ToString (CultureInfo.InvariantCulture));
+							builder.Write (buf, 0, buf.Length);
+
+							if (CanUseNonSynchronizedLiteral (Engine, length)) {
+								builder.WriteByte ((byte) '+');
+								wait = false;
+							}
+
+							builder.Write (LiteralTokenSuffix, 0, LiteralTokenSuffix.Length);
+
+							totalSize += length;
+
+							parts.Add (new ImapCommandPart (builder.ToArray (), literal, wait));
+							builder.SetLength (0);
+
+							if (prefix == UTF8LiteralTokenPrefix)
+								builder.WriteByte ((byte) ')');
+							break;
+						case 'S': // a string which may need to be quoted or made into a literal
+							AppendString (options, true, builder, (string) args[argc++]);
+							break;
+						case 'Q': // similar to %S but string must be quoted at a minimum
+							AppendString (options, false, builder, (string) args[argc++]);
+							break;
+						default:
+							throw new FormatException ();
+						}
+					} else if (format[i] < 128) {
+						builder.WriteByte ((byte) format[i]);
+					} else {
+						int nchars = char.IsSurrogate (format[i]) ? 2 : 1;
+						int nbytes = Encoding.UTF8.GetBytes (format, i, nchars, utf8, 0);
+						builder.Write (utf8, 0, nbytes);
+						i += nchars - 1;
+					}
+				}
+
+				parts.Add (new ImapCommandPart (builder.ToArray (), null));
+			}
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapCommand"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.Net.Imap.ImapCommand"/>.
+		/// </remarks>
+		/// <param name="engine">The IMAP engine that will be sending the command.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <param name="folder">The IMAP folder that the command operates on.</param>
+		/// <param name="format">The command format.</param>
+		/// <param name="args">The command arguments.</param>
+		public ImapCommand (ImapEngine engine, CancellationToken cancellationToken, ImapFolder folder, string format, params object[] args)
+			: this (engine, cancellationToken, folder, FormatOptions.Default, format, args)
+		{
+		}
+
+		internal static int EstimateCommandLength (ImapEngine engine, FormatOptions options, string format, params object[] args)
+		{
+			const int EstimatedTagLength = 10;
+			var eoln = false;
+			int length = 0;
+			int argc = 0;
+			string str;
+
+			for (int i = 0; i < format.Length; i++) {
+				if (format[i] == '%') {
+					switch (format[++i]) {
+					case '%': // a literal %
+						length++;
+						break;
+					case 'd': // an integer
+						str = ((int) args[argc++]).ToString (CultureInfo.InvariantCulture);
+						length += str.Length;
+						break;
+					case 'u': // an unsigned integer
+						str = ((uint) args[argc++]).ToString (CultureInfo.InvariantCulture);
+						length += str.Length;
+						break;
+					case 's':
+						str = (string) args[argc++];
+						length += str.Length;
+						break;
+					case 'F': // an ImapFolder
+						var utf7 = ((ImapFolder) args[argc++]).EncodedName;
+						length += EstimateStringLength (engine, options, true, utf7, out eoln);
+						break;
+					case 'L': // a MimeMessage or a byte[]
+						var arg = args[argc++];
+						byte[] prefix;
+						long len;
+
+						if (arg is MimeMessage message) {
+							prefix = options.International ? UTF8LiteralTokenPrefix : LiteralTokenPrefix;
+							var literal = new ImapLiteral (options, message, null);
+							len = literal.Length;
+						} else {
+							len = ((byte[]) arg).Length;
+							prefix = LiteralTokenPrefix;
+						}
+
+						length += prefix.Length;
+						length += Encoding.ASCII.GetByteCount (len.ToString (CultureInfo.InvariantCulture));
+
+						if (CanUseNonSynchronizedLiteral (engine, len))
+							length++;
+
+						length += LiteralTokenSuffix.Length;
+
+						if (prefix == UTF8LiteralTokenPrefix)
+							length++;
+
+						eoln = true;
+						break;
+					case 'S': // a string which may need to be quoted or made into a literal
+						length += EstimateStringLength (engine, options, true, (string) args[argc++], out eoln);
+						break;
+					case 'Q': // similar to %S but string must be quoted at a minimum
+						length += EstimateStringLength (engine, options, false, (string) args[argc++], out eoln);
+						break;
+					default:
+						throw new FormatException ();
+					}
+
+					if (eoln)
+						break;
+				} else {
+					length++;
+				}
+			}
+
+			return length + EstimatedTagLength;
+		}
+
+		internal static int EstimateCommandLength (ImapEngine engine, string format, params object[] args)
+		{
+			return EstimateCommandLength (engine, FormatOptions.Default, format, args);
+		}
+
+		void UpdateProgress (int n)
+		{
+			nwritten += n;
+
+			if (Progress != null)
+				Progress.Report (nwritten, totalSize);
+		}
+
+		static bool IsAtom (char c)
+		{
+			return c < 128 && !char.IsControl (c) && "(){ \t%*\\\"]".IndexOf (c) == -1;
+		}
+
+		static bool IsQuotedSafe (ImapEngine engine, char c)
+		{
+			return (c < 128 || engine.UTF8Enabled) && !char.IsControl (c);
+		}
+
+		internal static ImapStringType GetStringType (ImapEngine engine, string value, bool allowAtom)
+		{
+			var type = allowAtom ? ImapStringType.Atom : ImapStringType.QString;
+
+			if (value == null)
+				return ImapStringType.Nil;
+
+			if (value.Length == 0)
+				return ImapStringType.QString;
+
+			for (int i = 0; i < value.Length; i++) {
+				if (!IsAtom (value[i])) {
+					if (!IsQuotedSafe (engine, value[i]))
+						return ImapStringType.Literal;
+
+					type = ImapStringType.QString;
+				}
+			}
+
+			return type;
+		}
+
+		static bool CanUseNonSynchronizedLiteral (ImapEngine engine, long length)
+		{
+			return (engine.Capabilities & ImapCapabilities.LiteralPlus) != 0 ||
+				(length <= 4096 && (engine.Capabilities & ImapCapabilities.LiteralMinus) != 0);
+		}
+
+		static int EstimateStringLength (ImapEngine engine, FormatOptions options, bool allowAtom, string value, out bool eoln)
+		{
+			eoln = false;
+
+			switch (GetStringType (engine, value, allowAtom)) {
+			case ImapStringType.Literal:
+				var literal = Encoding.UTF8.GetByteCount (value);
+				var plus = CanUseNonSynchronizedLiteral (engine, literal);
+				int length = "{}\r\n".Length;
+
+				length += literal.ToString (CultureInfo.InvariantCulture).Length;
+				if (plus)
+					length++;
+
+				eoln = true;
+
+				return length++;
+			case ImapStringType.QString:
+				return Encoding.UTF8.GetByteCount (MimeUtils.Quote (value));
+			case ImapStringType.Nil:
+				return Nil.Length;
+			default:
+				return value.Length;
+			}
+		}
+
+		void AppendString (FormatOptions options, bool allowAtom, MemoryStream builder, string value)
+		{
+			byte[] buf;
+
+			switch (GetStringType (Engine, value, allowAtom)) {
+			case ImapStringType.Literal:
+				var literal = Encoding.UTF8.GetBytes (value);
+				var plus = CanUseNonSynchronizedLiteral (Engine, literal.Length);
+				var length = literal.Length.ToString (CultureInfo.InvariantCulture);
+				buf = Encoding.ASCII.GetBytes (length);
+
+				builder.WriteByte ((byte) '{');
+				builder.Write (buf, 0, buf.Length);
+				if (plus)
+					builder.WriteByte ((byte) '+');
+				builder.WriteByte ((byte) '}');
+				builder.WriteByte ((byte) '\r');
+				builder.WriteByte ((byte) '\n');
+
+				if (plus) {
+					builder.Write (literal, 0, literal.Length);
+				} else {
+					parts.Add (new ImapCommandPart (builder.ToArray (), new ImapLiteral (options, literal)));
+					builder.SetLength (0);
+				}
+				break;
+			case ImapStringType.QString:
+				buf = Encoding.UTF8.GetBytes (MimeUtils.Quote (value));
+				builder.Write (buf, 0, buf.Length);
+				break;
+			case ImapStringType.Atom:
+				buf = Encoding.UTF8.GetBytes (value);
+				builder.Write (buf, 0, buf.Length);
+				break;
+			case ImapStringType.Nil:
+				builder.Write (Nil, 0, Nil.Length);
+				break;
+			}
+		}
+
+		/// <summary>
+		/// Registers the untagged handler for the specified atom token.
+		/// </summary>
+		/// <param name="atom">The atom token.</param>
+		/// <param name="handler">The handler.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="atom"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="handler"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Untagged handlers must be registered before the command has been queued.
+		/// </exception>
+		public void RegisterUntaggedHandler (string atom, ImapUntaggedHandler handler)
+		{
+			if (atom == null)
+				throw new ArgumentNullException (nameof (atom));
+
+			if (handler == null)
+				throw new ArgumentNullException (nameof (handler));
+
+			if (Status != ImapCommandStatus.Created)
+				throw new InvalidOperationException ("Untagged handlers must be registered before the command has been queued.");
+
+			UntaggedHandlers.Add (atom, handler);
+		}
+
+		/// <summary>
+		/// Sends the next part of the command to the server.
+		/// </summary>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// An IMAP protocol error occurred.
+		/// </exception>
+		public async Task<bool> StepAsync (bool doAsync)
+		{
+			var supportsLiteralPlus = (Engine.Capabilities & ImapCapabilities.LiteralPlus) != 0;
+			var idle = UserData as ImapIdleContext;
+			var result = ImapCommandResponse.None;
+			ImapToken token;
+
+			// construct and write the command tag if this is the initial state
+			if (current == 0) {
+				Tag = string.Format (CultureInfo.InvariantCulture, "{0}{1:D8}", Engine.TagPrefix, Engine.Tag++);
+
+				var buf = Encoding.ASCII.GetBytes (Tag + " ");
+
+				if (doAsync)
+					await Engine.Stream.WriteAsync (buf, 0, buf.Length, CancellationToken).ConfigureAwait (false);
+				else
+					Engine.Stream.Write (buf, 0, buf.Length, CancellationToken);
+			}
+
+			do {
+				var command = parts[current].Command;
+
+				if (doAsync)
+					await Engine.Stream.WriteAsync (command, 0, command.Length, CancellationToken).ConfigureAwait (false);
+				else
+					Engine.Stream.Write (command, 0, command.Length, CancellationToken);
+
+				// if the server doesn't support LITERAL+ (or LITERAL-), we'll need to wait
+				// for a "+" response before writing out the any literals...
+				if (parts[current].WaitForContinuation)
+					break;
+
+				// otherwise, we can write out any and all literal tokens we have...
+				await parts[current].Literal.WriteToAsync (Engine.Stream, doAsync, CancellationToken).ConfigureAwait (false);
+
+				if (current + 1 >= parts.Count)
+					break;
+
+				current++;
+			} while (true);
+
+			if (doAsync)
+				await Engine.Stream.FlushAsync (CancellationToken).ConfigureAwait (false);
+			else
+				Engine.Stream.Flush (CancellationToken);
+
+			// now we need to read the response...
+			do {
+				if (Engine.State == ImapEngineState.Idle) {
+					int timeout = Timeout.Infinite;
+
+					if (Engine.Stream.CanTimeout) {
+						timeout = Engine.Stream.ReadTimeout;
+						Engine.Stream.ReadTimeout = Timeout.Infinite;
+					}
+
+					try {
+						token = await Engine.ReadTokenAsync (doAsync, CancellationToken).ConfigureAwait (false);
+					} finally {
+						if (Engine.Stream.IsConnected && Engine.Stream.CanTimeout)
+							Engine.Stream.ReadTimeout = timeout;
+					}
+				} else {
+					token = await Engine.ReadTokenAsync (doAsync, CancellationToken).ConfigureAwait (false);
+				}
+
+				if (token.Type == ImapTokenType.Atom && token.Value.ToString () == "+") {
+					// we've gotten a continuation response from the server
+					var text = (await Engine.ReadLineAsync (doAsync, CancellationToken).ConfigureAwait (false)).Trim ();
+
+					// if we've got a Literal pending, the '+' means we can send it now...
+					if (!supportsLiteralPlus && parts[current].Literal != null) {
+						await parts[current].Literal.WriteToAsync (Engine.Stream, doAsync, CancellationToken).ConfigureAwait (false);
+						break;
+					}
+
+					if (ContinuationHandler != null) {
+						await ContinuationHandler (Engine, this, text, doAsync).ConfigureAwait (false);
+					} else if (doAsync) {
+						await Engine.Stream.WriteAsync (NewLine, 0, NewLine.Length, CancellationToken).ConfigureAwait (false);
+						await Engine.Stream.FlushAsync (CancellationToken).ConfigureAwait (false);
+					} else {
+						Engine.Stream.Write (NewLine, 0, NewLine.Length, CancellationToken);
+						Engine.Stream.Flush (CancellationToken);
+					}
+				} else if (token.Type == ImapTokenType.Asterisk) {
+					// we got an untagged response, let the engine handle this...
+					await Engine.ProcessUntaggedResponseAsync (doAsync, CancellationToken).ConfigureAwait (false);
+				} else if (token.Type == ImapTokenType.Atom && (string) token.Value == Tag) {
+					// the next token should be "OK", "NO", or "BAD"
+					token = await Engine.ReadTokenAsync (doAsync, CancellationToken).ConfigureAwait (false);
+
+					ImapEngine.AssertToken (token, ImapTokenType.Atom, "Syntax error in tagged response. Unexpected token: {0}", token);
+
+					string atom = (string) token.Value;
+
+					switch (atom) {
+					case "BAD": result = ImapCommandResponse.Bad; break;
+					case "OK": result = ImapCommandResponse.Ok; break;
+					case "NO": result = ImapCommandResponse.No; break;
+					default: throw ImapEngine.UnexpectedToken ("Syntax error in tagged response. Unexpected token: {0}", token);
+					}
+
+					token = await Engine.ReadTokenAsync (doAsync, CancellationToken).ConfigureAwait (false);
+					if (token.Type == ImapTokenType.OpenBracket) {
+						var code = await Engine.ParseResponseCodeAsync (true, doAsync, CancellationToken).ConfigureAwait (false);
+						RespCodes.Add (code);
+						break;
+					}
+
+					if (token.Type != ImapTokenType.Eoln) {
+						// consume the rest of the line...
+						var line = await Engine.ReadLineAsync (doAsync, CancellationToken).ConfigureAwait (false);
+						ResponseText = (((string) token.Value) + line).TrimEnd ();
+						break;
+					}
+				} else if (token.Type == ImapTokenType.OpenBracket) {
+					// Note: this is a work-around for broken IMAP servers like Office365.com that
+					// return RESP-CODES that are not preceded by "* OK " such as the example in
+					// issue #115 (https://github.com/jstedfast/MailKit/issues/115).
+					var code = await Engine.ParseResponseCodeAsync (false, doAsync, CancellationToken).ConfigureAwait (false);
+					RespCodes.Add (code);
+				} else {
+					// no clue what we got...
+					throw ImapEngine.UnexpectedToken ("Syntax error in response. Unexpected token: {0}", token);
+				}
+			} while (Status == ImapCommandStatus.Active);
+
+			if (Status == ImapCommandStatus.Active) {
+				current++;
+
+				if (current >= parts.Count || result != ImapCommandResponse.None) {
+					Status = ImapCommandStatus.Complete;
+					Response = result;
+					return false;
+				}
+
+				return true;
+			}
+
+			return false;
+		}
+	}
+}

src/MailKit/Net/Imap/ImapCommandException.cs

@@ -0,0 +1,190 @@
+//
+// ImapCommandException.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+#if SERIALIZABLE
+using System.Security;
+using System.Runtime.Serialization;
+#endif
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// An exception that is thrown when an IMAP command returns NO or BAD.
+	/// </summary>
+	/// <remarks>
+	/// The exception that is thrown when an IMAP command fails. Unlike a <see cref="ImapProtocolException"/>,
+	/// a <see cref="ImapCommandException"/> does not require the <see cref="ImapClient"/> to be reconnected.
+	/// </remarks>
+#if SERIALIZABLE
+	[Serializable]
+#endif
+	public class ImapCommandException : CommandException
+	{
+#if SERIALIZABLE
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapCommandException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ImapCommandException"/> from the serialized data.
+		/// </remarks>
+		/// <param name="info">The serialization info.</param>
+		/// <param name="context">The streaming context.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="info"/> is <c>null</c>.
+		/// </exception>
+		[SecuritySafeCritical]
+		protected ImapCommandException (SerializationInfo info, StreamingContext context) : base (info, context)
+		{
+			Response = (ImapCommandResponse) info.GetValue ("Response", typeof (ImapCommandResponse));
+			ResponseText = info.GetString ("ResponseText");
+		}
+#endif
+
+		/// <summary>
+		/// Create a new <see cref="ImapCommandException"/> based on the specified command name and <see cref="ImapCommand"/> state.
+		/// </summary>
+		/// <remarks>
+		/// Create a new <see cref="ImapCommandException"/> based on the specified command name and <see cref="ImapCommand"/> state.
+		/// </remarks>
+		/// <returns>A new command exception.</returns>
+		/// <param name="command">The command name.</param>
+		/// <param name="ic">The command state.</param>
+		internal static ImapCommandException Create (string command, ImapCommand ic)
+		{
+			var result = ic.Response.ToString ().ToUpperInvariant ();
+			string message, reason = null;
+
+			if (string.IsNullOrEmpty (ic.ResponseText)) {
+				for (int i = ic.RespCodes.Count - 1; i >= 0; i--) {
+					if (ic.RespCodes[i].IsError && !string.IsNullOrEmpty (ic.RespCodes[i].Message)) {
+						reason = ic.RespCodes[i].Message;
+						break;
+					}
+				}
+			} else {
+				reason = ic.ResponseText;
+			}
+
+			if (!string.IsNullOrEmpty (reason))
+				message = string.Format ("The IMAP server replied to the '{0}' command with a '{1}' response: {2}", command, result, reason);
+			else
+				message = string.Format ("The IMAP server replied to the '{0}' command with a '{1}' response.", command, result);
+
+			return ic.Exception != null ? new ImapCommandException (ic.Response, reason, message, ic.Exception) : new ImapCommandException (ic.Response, reason, message);
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapCommandException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ImapCommandException"/>.
+		/// </remarks>
+		/// <param name="response">The IMAP command response.</param>
+		/// <param name="message">The error message.</param>
+		/// <param name="responseText">The human-readable response text.</param>
+		/// <param name="innerException">The inner exception.</param>
+		public ImapCommandException (ImapCommandResponse response, string responseText, string message, Exception innerException) : base (message, innerException)
+		{
+			ResponseText = responseText;
+			Response = response;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapCommandException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ImapCommandException"/>.
+		/// </remarks>
+		/// <param name="response">The IMAP command response.</param>
+		/// <param name="responseText">The human-readable response text.</param>
+		/// <param name="message">The error message.</param>
+		public ImapCommandException (ImapCommandResponse response, string responseText, string message) : base (message)
+		{
+			ResponseText = responseText;
+			Response = response;
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapCommandException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ImapCommandException"/>.
+		/// </remarks>
+		/// <param name="response">The IMAP command response.</param>
+		/// <param name="responseText">The human-readable response text.</param>
+		public ImapCommandException (ImapCommandResponse response, string responseText)
+		{
+			ResponseText = responseText;
+			Response = response;
+		}
+
+		/// <summary>
+		/// Gets the IMAP command response.
+		/// </summary>
+		/// <remarks>
+		/// Gets the IMAP command response.
+		/// </remarks>
+		/// <value>The IMAP command response.</value>
+		public ImapCommandResponse Response {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Gets the human-readable IMAP command response text.
+		/// </summary>
+		/// <remarks>
+		/// Gets the human-readable IMAP command response text.
+		/// </remarks>
+		/// <value>The response text.</value>
+		public string ResponseText {
+			get; private set;
+		}
+
+#if SERIALIZABLE
+		/// <summary>
+		/// When overridden in a derived class, sets the <see cref="System.Runtime.Serialization.SerializationInfo"/>
+		/// with information about the exception.
+		/// </summary>
+		/// <remarks>
+		/// Serializes the state of the <see cref="FolderNotFoundException"/>.
+		/// </remarks>
+		/// <param name="info">The serialization info.</param>
+		/// <param name="context">The streaming context.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="info"/> is <c>null</c>.
+		/// </exception>
+		[SecurityCritical]
+		public override void GetObjectData (SerializationInfo info, StreamingContext context)
+		{
+			base.GetObjectData (info, context);
+
+			info.AddValue ("Response", Response, typeof (ImapCommandResponse));
+			info.AddValue ("ResponseText", ResponseText);
+		}
+#endif
+	}
+}

src/MailKit/Net/Imap/ImapCommandResponse.cs

@@ -0,0 +1,55 @@
+//
+// ImapCommandResult.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// An enumeration of possible IMAP command responses.
+	/// </summary>
+	/// <remarks>
+	/// An enumeration of possible IMAP command responses.
+	/// </remarks>
+	public enum ImapCommandResponse {
+		/// <summary>
+		/// No IMAP command response yet.
+		/// </summary>
+		None,
+
+		/// <summary>
+		/// The command resulted in an "OK" response.
+		/// </summary>
+		Ok,
+
+		/// <summary>
+		/// The command resulted in a "NO" response.
+		/// </summary>
+		No,
+
+		/// <summary>
+		/// The command resulted in a "BAD" response.
+		/// </summary>
+		Bad
+	}
+}

src/MailKit/Net/Imap/ImapEncoding.cs

@@ -0,0 +1,155 @@
+//
+// ImapEncoding.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Text;
+
+namespace MailKit.Net.Imap {
+	static class ImapEncoding
+	{
+		const string utf7_alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,";
+
+		static readonly byte[] utf7_rank = {
+			255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,
+			255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,
+			255,255,255,255,255,255,255,255,255,255,255, 62, 63,255,255,255,
+			 52, 53, 54, 55, 56, 57, 58, 59, 60, 61,255,255,255,255,255,255,
+			255,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
+			 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,255,255,255,255,255,
+			255, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+			 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51,255,255,255,255,255,
+		};
+
+		public static string Decode (string text)
+		{
+			var decoded = new StringBuilder ();
+			bool shifted = false;
+			int bits = 0, v = 0;
+			int index = 0;
+			char c;
+
+			while (index < text.Length) {
+				c = text[index++];
+
+				if (shifted) {
+					if (c == '-') {
+						// shifted back out of modified UTF-7
+						shifted = false;
+						bits = v = 0;
+					} else if (c > 127) {
+						// invalid UTF-7
+						return text;
+					} else {
+						byte rank = utf7_rank[(byte) c];
+
+						if (rank == 0xff) {
+							// invalid UTF-7
+							return text;
+						}
+
+						v = (v << 6) | rank;
+						bits += 6;
+
+						if (bits >= 16) {
+							char u = (char) ((v >> (bits - 16)) & 0xffff);
+							decoded.Append (u);
+							bits -= 16;
+						}
+					}
+				} else if (c == '&' && index < text.Length) {
+					if (text[index] == '-') {
+						decoded.Append ('&');
+						index++;
+					} else {
+						// shifted into modified UTF-7
+						shifted = true;
+					}
+				} else {
+					decoded.Append (c);
+				}
+			}
+
+			return decoded.ToString ();
+		}
+
+		static void Utf7ShiftOut (StringBuilder output, int u, int bits)
+		{
+			if (bits > 0) {
+				int x = (u << (6 - bits)) & 0x3f;
+				output.Append (utf7_alphabet[x]);
+			}
+
+			output.Append ('-');
+		}
+
+		public static string Encode (string text)
+		{
+			var encoded = new StringBuilder ();
+			bool shifted = false;
+			int bits = 0, u = 0;
+
+			for (int index = 0; index < text.Length; index++) {
+				char c = text[index];
+
+				if (c >= 0x20 && c < 0x7f) {
+					// characters with octet values 0x20-0x25 and 0x27-0x7e
+					// represent themselves while 0x26 ("&") is represented
+					// by the two-octet sequence "&-"
+
+					if (shifted) {
+						Utf7ShiftOut (encoded, u, bits);
+						shifted = false;
+						bits = 0;
+					}
+
+					if (c == 0x26)
+						encoded.Append ("&-");
+					else
+						encoded.Append (c);
+				} else {
+					// base64 encode
+					if (!shifted) {
+						encoded.Append ('&');
+						shifted = true;
+					}
+
+					u = (u << 16) | (c & 0xffff);
+					bits += 16;
+
+					while (bits >= 6) {
+						int x = (u >> (bits - 6)) & 0x3f;
+						encoded.Append (utf7_alphabet[x]);
+						bits -= 6;
+					}
+				}
+			}
+
+			if (shifted)
+				Utf7ShiftOut (encoded, u, bits);
+
+			return encoded.ToString ();
+		}
+	}
+}

src/MailKit/Net/Imap/ImapEventGroup.cs

@@ -0,0 +1,692 @@
+//
+// ImapFolderFetch.cs
+//
+// Authors: Steffen Kieß <s-kiess@web.de>
+//          Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+using System.Collections.Generic;
+
+using MimeKit;
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// An IMAP event group used with the NOTIFY command.
+	/// </summary>
+	/// <remarks>
+	/// An IMAP event group used with the NOTIFY command.
+	/// </remarks>
+	public sealed class ImapEventGroup
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEventGroup"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEventGroup"/> class.
+		/// </remarks>
+		/// <param name="mailboxFilter">The mailbox filter.</param>
+		/// <param name="events">The list of IMAP events.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="mailboxFilter"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="events"/> is <c>null</c>.</para>
+		/// </exception>
+		public ImapEventGroup (ImapMailboxFilter mailboxFilter, IList<ImapEvent> events)
+		{
+			if (mailboxFilter == null)
+				throw new ArgumentNullException (nameof (mailboxFilter));
+
+			if (events == null)
+				throw new ArgumentNullException (nameof (events));
+
+			MailboxFilter = mailboxFilter;
+			Events = events;
+		}
+
+		/// <summary>
+		/// Get the mailbox filter.
+		/// </summary>
+		/// <remarks>
+		/// Gets the mailbox filter.
+		/// </remarks>
+		/// <value>The mailbox filter.</value>
+		public ImapMailboxFilter MailboxFilter {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the list of IMAP events.
+		/// </summary>
+		/// <remarks>
+		/// Gets the list of IMAP events.
+		/// </remarks>
+		/// <value>The events.</value>
+		public IList<ImapEvent> Events {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Format the IMAP NOTIFY command for this particular IMAP event group.
+		/// </summary>
+		/// <remarks>
+		/// Formats the IMAP NOTIFY command for this particular IMAP event group.
+		/// </remarks>
+		/// <param name="engine">The IMAP engine.</param>
+		/// <param name="command">The IMAP command builder.</param>
+		/// <param name="args">The IMAP command argument builder.</param>
+		/// <param name="notifySelectedNewExpunge">Gets set to <c>true</c> if the NOTIFY command requests the MessageNew or
+		/// MessageExpunged events for a SELECTED or SELECTED-DELAYED mailbox filter; otherwise it is left unchanged.</param>
+		internal void Format (ImapEngine engine, StringBuilder command, IList<object> args, ref bool notifySelectedNewExpunge)
+		{
+			bool isSelectedFilter = MailboxFilter == ImapMailboxFilter.Selected || MailboxFilter == ImapMailboxFilter.SelectedDelayed;
+
+			command.Append ("(");
+			MailboxFilter.Format (engine, command, args);
+			command.Append (" ");
+
+			if (Events.Count > 0) {
+				var haveAnnotationChange = false;
+				var haveMessageExpunge = false;
+				var haveMessageNew = false;
+				var haveFlagChange = false;
+
+				command.Append ("(");
+
+				for (int i = 0; i < Events.Count; i++) {
+					var @event = Events[i];
+
+					if (isSelectedFilter && !@event.IsMessageEvent)
+						throw new InvalidOperationException ("Only message events may be specified when SELECTED or SELECTED-DELAYED is used.");
+
+					if (@event is ImapEvent.MessageNew)
+						haveMessageNew = true;
+					else if (@event == ImapEvent.MessageExpunge)
+						haveMessageExpunge = true;
+					else if (@event == ImapEvent.FlagChange)
+						haveFlagChange = true;
+					else if (@event == ImapEvent.AnnotationChange)
+						haveAnnotationChange = true;
+
+					if (i > 0)
+						command.Append (" ");
+
+					@event.Format (engine, command, args, isSelectedFilter);
+				}
+				command.Append (")");
+
+				// https://tools.ietf.org/html/rfc5465#section-5
+				if ((haveMessageNew && !haveMessageExpunge) || (!haveMessageNew && haveMessageExpunge))
+					throw new InvalidOperationException ("If MessageNew or MessageExpunge is specified, both must be specified.");
+
+				if ((haveFlagChange || haveAnnotationChange) && (!haveMessageNew || !haveMessageExpunge))
+					throw new InvalidOperationException ("If FlagChange and/or AnnotationChange are specified, MessageNew and MessageExpunge must also be specified.");
+
+				notifySelectedNewExpunge = (haveMessageNew || haveMessageExpunge) && MailboxFilter == ImapMailboxFilter.Selected;
+			} else {
+				command.Append ("NONE");
+			}
+
+			command.Append (")");
+		}
+	}
+
+	/// <summary>
+	/// An IMAP mailbox filter for use with the NOTIFY command.
+	/// </summary>
+	/// <remarks>
+	/// An IMAP mailbox filter for use with the NOTIFY command.
+	/// </remarks>
+	public class ImapMailboxFilter
+	{
+		/// <summary>
+		/// An IMAP mailbox filter specifying that the client wants immediate notifications for
+		/// the currently selected folder.
+		/// </summary>
+		/// <remarks>
+		/// The <c>SELECTED</c> mailbox specifier requires the server to send immediate
+		/// notifications for the currently selected mailbox about all specified
+		/// message events.
+		/// </remarks>
+		public static readonly ImapMailboxFilter Selected = new ImapMailboxFilter ("SELECTED");
+
+		/// <summary>
+		/// An IMAP mailbox filter specifying the currently selected folder but delays notifications
+		/// until a command has been issued.
+		/// </summary>
+		/// <remarks>
+		/// The <c>SELECTED-DELAYED</c> mailbox specifier requires the server to delay a
+		/// <see cref="ImapEvent.MessageExpunge"/> event until the client issues a command that allows
+		/// returning information about expunged messages (see
+		/// <a href="https://tools.ietf.org/html/rfc3501#section-7.4.1">Section 7.4.1 of RFC3501]</a>
+		/// for more details), for example, till a <c>NOOP</c> or an <c>IDLE</c> command has been issued.
+		/// When <c>SELECTED-DELAYED</c> is specified, the server MAY also delay returning other message
+		/// events until the client issues one of the commands specified above, or it MAY return them
+		/// immediately.
+		/// </remarks>
+		public static readonly ImapMailboxFilter SelectedDelayed = new ImapMailboxFilter ("SELECTED-DELAYED");
+
+		/// <summary>
+		/// An IMAP mailbox filter specifying the currently selected folder.
+		/// </summary>
+		/// <remarks>
+		/// <para>The <c>INBOXES</c> mailbox specifier refers to all selectable mailboxes in the user's
+		/// personal namespace(s) to which messages may be delivered by a Message Delivery Agent (MDA).
+		/// </para>
+		/// <para>If the IMAP server cannot easily compute this set, it MUST treat <see cref="Inboxes"/>
+		/// as equivalent to <see cref="Personal"/>.</para>
+		/// </remarks>
+		public static readonly ImapMailboxFilter Inboxes = new ImapMailboxFilter ("INBOXES");
+
+		/// <summary>
+		/// An IMAP mailbox filter specifying all selectable folders within the user's personal namespace.
+		/// </summary>
+		/// <remarks>
+		/// The <c>PERSONAL</c> mailbox specifier refers to all selectable folders within the user's personal namespace.
+		/// </remarks>
+		public static readonly ImapMailboxFilter Personal = new ImapMailboxFilter ("PERSONAL");
+
+		/// <summary>
+		/// An IMAP mailbox filter that refers to all subscribed folders.
+		/// </summary>
+		/// <remarks>
+		/// <para>The <c>SUBSCRIBED</c> mailbox specifier refers to all folders subscribed to by the user.</para>
+		/// <para>If the subscription list changes, the server MUST reevaluate the list.</para>
+		/// </remarks>
+		public static readonly ImapMailboxFilter Subscribed = new ImapMailboxFilter ("SUBSCRIBED");
+
+		/// <summary>
+		/// An IMAP mailbox filter that specifies a list of folders to receive notifications about.
+		/// </summary>
+		/// <remarks>
+		/// An IMAP mailbox filter that specifies a list of folders to receive notifications about.
+		/// </remarks>
+		public class Mailboxes : ImapMailboxFilter
+		{
+			readonly ImapFolder[] folders;
+
+			/// <summary>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Mailboxes"/> class.
+			/// </summary>
+			/// <remarks>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Mailboxes"/> class.
+			/// </remarks>
+			/// <param name="folders">The list of folders to watch for events.</param>
+			/// <exception cref="System.ArgumentNullException">
+			/// <paramref name="folders"/> is <c>null</c>.
+			/// </exception>
+			/// <exception cref="System.ArgumentException">
+			/// <para>The list of <paramref name="folders"/> is empty.</para>
+			/// <para>-or-</para>
+			/// <para>The list of <paramref name="folders"/> contains folders that are not of
+			/// type <see cref="ImapFolder"/>.</para>
+			/// </exception>
+			public Mailboxes (IList<IMailFolder> folders) : this ("MAILBOXES", folders)
+			{
+			}
+
+			/// <summary>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Mailboxes"/> class.
+			/// </summary>
+			/// <remarks>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Mailboxes"/> class.
+			/// </remarks>
+			/// <param name="folders">The list of folders to watch for events.</param>
+			/// <exception cref="System.ArgumentNullException">
+			/// <paramref name="folders"/> is <c>null</c>.
+			/// </exception>
+			/// <exception cref="System.ArgumentException">
+			/// <para>The list of <paramref name="folders"/> is empty.</para>
+			/// <para>-or-</para>
+			/// <para>The list of <paramref name="folders"/> contains folders that are not of
+			/// type <see cref="ImapFolder"/>.</para>
+			/// </exception>
+			public Mailboxes (params IMailFolder[] folders) : this ("MAILBOXES", folders)
+			{
+			}
+
+			/// <summary>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Mailboxes"/> class.
+			/// </summary>
+			/// <remarks>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Mailboxes"/> class.
+			/// </remarks>
+			/// <param name="name">The name of the mailbox filter.</param>
+			/// <param name="folders">The list of folders to watch for events.</param>
+			/// <exception cref="System.ArgumentNullException">
+			/// <paramref name="folders"/> is <c>null</c>.
+			/// </exception>
+			/// <exception cref="System.ArgumentException">
+			/// <para>The list of <paramref name="folders"/> is empty.</para>
+			/// <para>-or-</para>
+			/// <para>The list of <paramref name="folders"/> contains folders that are not of
+			/// type <see cref="ImapFolder"/>.</para>
+			/// </exception>
+			internal Mailboxes (string name, IList<IMailFolder> folders) : base (name)
+			{
+				if (folders == null)
+					throw new ArgumentNullException (nameof (folders));
+
+				if (folders.Count == 0)
+					throw new ArgumentException ("Must supply at least one folder.", nameof (folders));
+
+				this.folders = new ImapFolder[folders.Count];
+				for (int i = 0; i < folders.Count; i++) {
+					if (!(folders[i] is ImapFolder folder))
+						throw new ArgumentException ("All folders must be ImapFolders.", nameof (folders));
+
+					this.folders[i] = folder;
+				}
+			}
+
+			/// <summary>
+			/// Format the IMAP NOTIFY command for this particular IMAP mailbox filter.
+			/// </summary>
+			/// <remarks>
+			/// Formats the IMAP NOTIFY command for this particular IMAP mailbox filter.
+			/// </remarks>
+			/// <param name="engine">The IMAP engine.</param>
+			/// <param name="command">The IMAP command builder.</param>
+			/// <param name="args">The IMAP command argument builder.</param>
+			internal override void Format (ImapEngine engine, StringBuilder command, IList<object> args)
+			{
+				command.Append (Name);
+				command.Append (' ');
+
+				// FIXME: should we verify that each ImapFolder belongs to this ImapEngine?
+
+				if (folders.Length == 1) {
+					command.Append ("%F");
+					args.Add (folders[0]);
+				} else {
+					command.Append ("(");
+
+					for (int i = 0; i < folders.Length; i++) {
+						if (i > 0)
+							command.Append (" ");
+						command.Append ("%F");
+						args.Add (folders[i]);
+					}
+
+					command.Append (")");
+				}
+			}
+		}
+
+		/// <summary>
+		/// An IMAP mailbox filter that specifies a list of folder subtrees to get notifications about.
+		/// </summary>
+		/// <remarks>
+		/// <para>The client will receive notifications for each specified folder plus all selectable
+		/// folders that are subordinate to any of the specified folders.</para>
+		/// </remarks>
+		public class Subtree : Mailboxes
+		{
+			/// <summary>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Subtree"/> class.
+			/// </summary>
+			/// <remarks>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Subtree"/> class.
+			/// </remarks>
+			/// <param name="folders">The list of folders to watch for events.</param>
+			/// <exception cref="System.ArgumentNullException">
+			/// <paramref name="folders"/> is <c>null</c>.
+			/// </exception>
+			/// <exception cref="System.ArgumentException">
+			/// <para>The list of <paramref name="folders"/> is empty.</para>
+			/// <para>-or-</para>
+			/// <para>The list of <paramref name="folders"/> contains folders that are not of
+			/// type <see cref="ImapFolder"/>.</para>
+			/// </exception>
+			public Subtree (IList<IMailFolder> folders) : base ("SUBTREE", folders)
+			{
+			}
+
+			/// <summary>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Subtree"/> class.
+			/// </summary>
+			/// <remarks>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter.Subtree"/> class.
+			/// </remarks>
+			/// <param name="folders">The list of folders to watch for events.</param>
+			/// <exception cref="System.ArgumentNullException">
+			/// <paramref name="folders"/> is <c>null</c>.
+			/// </exception>
+			/// <exception cref="System.ArgumentException">
+			/// <para>The list of <paramref name="folders"/> is empty.</para>
+			/// <para>-or-</para>
+			/// <para>The list of <paramref name="folders"/> contains folders that are not of
+			/// type <see cref="ImapFolder"/>.</para>
+			/// </exception>
+			public Subtree (params IMailFolder[] folders) : base ("SUBTREE", folders)
+			{
+			}
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapMailboxFilter"/> class.
+		/// </remarks>
+		/// <param name="name">The name of the mailbox filter.</param>
+		internal ImapMailboxFilter (string name)
+		{
+			Name = name;
+		}
+
+		/// <summary>
+		/// Get the name of the mailbox filter.
+		/// </summary>
+		/// <remarks>
+		/// Gets the name of the mailbox filter.
+		/// </remarks>
+		/// <value>The name.</value>
+		public string Name { get; private set; }
+
+		/// <summary>
+		/// Format the IMAP NOTIFY command for this particular IMAP mailbox filter.
+		/// </summary>
+		/// <remarks>
+		/// Formats the IMAP NOTIFY command for this particular IMAP mailbox filter.
+		/// </remarks>
+		/// <param name="engine">The IMAP engine.</param>
+		/// <param name="command">The IMAP command builder.</param>
+		/// <param name="args">The IMAP command argument builder.</param>
+		internal virtual void Format (ImapEngine engine, StringBuilder command, IList<object> args)
+		{
+			command.Append (Name);
+		}
+	}
+
+	/// <summary>
+	/// An IMAP notification event.
+	/// </summary>
+	/// <remarks>
+	/// An IMAP notification event.
+	/// </remarks>
+	public class ImapEvent
+	{
+		/// <summary>
+		/// An IMAP event notification for expunged messages.
+		/// </summary>
+		/// <remarks>
+		/// <para>If the expunged message or messages are in the selected mailbox, the server notifies the client
+		/// using <see cref="IMailFolder.MessageExpunged"/> (or <see cref="IMailFolder.MessagesVanished"/> if
+		/// the <a href="https://tools.ietf.org/html/rfc5162">QRESYNC</a> extension has been enabled via
+		/// <see cref="ImapClient.EnableQuickResync(System.Threading.CancellationToken)"/> or
+		/// <see cref="ImapClient.EnableQuickResyncAsync(System.Threading.CancellationToken)"/>).</para>
+		/// <para>If the expunged message or messages are in another mailbox, the <see cref="IMailFolder.UidNext"/>
+		/// and <see cref="IMailFolder.Count"/> properties will be updated and the appropriate
+		/// <see cref="IMailFolder.UidNextChanged"/> and <see cref="IMailFolder.CountChanged"/> events will be
+		/// emitted for the relevant folder. If the <a href="https://tools.ietf.org/html/rfc5162">QRESYNC</a>
+		/// extension is enabled, the <see cref="IMailFolder.HighestModSeq"/> property will also be updated and
+		/// the <see cref="IMailFolder.HighestModSeqChanged"/> event will be emitted.</para>
+		/// <note type="note">if a client requests <see cref="MessageExpunge"/> with the <see cref="ImapMailboxFilter.Selected"/>
+		/// mailbox specifier, the meaning of a message index can change at any time, so the client cannot use
+		/// message indexes in commands anymore. The client MUST use API variants that take <see cref="UniqueId"/> or
+		/// a <see cref="IList{UniqueId}"/>. The meaning of <c>*</c>* can also change when messages are added or expunged.
+		/// A client wishing to keep using message indexes can either use the <see cref="ImapMailboxFilter.SelectedDelayed"/>
+		/// mailbox specifier or can avoid using the <see cref="MessageExpunge"/> event entirely.</note>
+		/// </remarks>
+		public static readonly ImapEvent MessageExpunge = new ImapEvent ("MessageExpunge", true);
+
+		/// <summary>
+		/// An IMAP event notification for message flag changes.
+		/// </summary>
+		/// <remarks>
+		/// <para>If the <see cref="FlagChange"/> notification arrives for a message located in the currently selected
+		/// folder, then that folder will emit a <see cref="IMailFolder.MessageFlagsChanged"/> event as well as a
+		/// <see cref="IMailFolder.MessageSummaryFetched"/> event with an appropriately populated
+		/// <see cref="IMessageSummary"/>.</para>
+		/// <para>On the other hand, if the <see cref="FlagChange"/> notification arrives for a message that is not
+		/// located in the currently selected folder, then the events that are emitted will depend on the
+		/// <see cref="ImapCapabilities"/> of the IMAP server.</para>
+		/// <para>If the server supports the <see cref="ImapCapabilities.CondStore"/> capability (or the
+		/// <see cref="ImapCapabilities.QuickResync"/> capability and the client has enabled it via
+		/// <see cref="ImapClient.EnableQuickResync(System.Threading.CancellationToken)"/>), then the
+		/// <see cref="IMailFolder.HighestModSeqChanged"/> event will be emitted as well as the
+		/// <see cref="IMailFolder.UidValidityChanged"/> event (if the latter has changed). If the number of
+		/// seen messages has changed, then the <see cref="IMailFolder.UnreadChanged"/> event may also be emitted.</para>
+		/// <para>If the server does not support either the <see cref="ImapCapabilities.CondStore"/> capability nor
+		/// the <see cref="ImapCapabilities.QuickResync"/> capability and the client has not enabled the later capability
+		/// via <see cref="ImapClient.EnableQuickResync(System.Threading.CancellationToken)"/>, then the server may choose
+		/// only to notify the client of <see cref="IMailFolder.UidValidity"/> changes by emitting the
+		/// <see cref="IMailFolder.UidValidityChanged"/> event.</para>
+		/// </remarks>
+		public static readonly ImapEvent FlagChange = new ImapEvent ("FlagChange", true);
+
+		/// <summary>
+		/// An IMAP event notification for message annotation changes.
+		/// </summary>
+		/// <remarks>
+		/// <para>If the <see cref="AnnotationChange"/> notification arrives for a message located in the currently selected
+		/// folder, then that folder will emit a <see cref="IMailFolder.AnnotationsChanged"/> event as well as a
+		/// <see cref="IMailFolder.MessageSummaryFetched"/> event with an appropriately populated
+		/// <see cref="IMessageSummary"/>.</para>
+		/// <para>On the other hand, if the <see cref="AnnotationChange"/> notification arrives for a message that is not
+		/// located in the currently selected folder, then the events that are emitted will depend on the
+		/// <see cref="ImapCapabilities"/> of the IMAP server.</para>
+		/// <para>If the server supports the <see cref="ImapCapabilities.CondStore"/> capability (or the
+		/// <see cref="ImapCapabilities.QuickResync"/> capability and the client has enabled it via
+		/// <see cref="ImapClient.EnableQuickResync(System.Threading.CancellationToken)"/>), then the
+		/// <see cref="IMailFolder.HighestModSeqChanged"/> event will be emitted as well as the
+		/// <see cref="IMailFolder.UidValidityChanged"/> event (if the latter has changed). If the number of
+		/// seen messages has changed, then the <see cref="IMailFolder.UnreadChanged"/> event may also be emitted.</para>
+		/// <para>If the server does not support either the <see cref="ImapCapabilities.CondStore"/> capability nor
+		/// the <see cref="ImapCapabilities.QuickResync"/> capability and the client has not enabled the later capability
+		/// via <see cref="ImapClient.EnableQuickResync(System.Threading.CancellationToken)"/>, then the server may choose
+		/// only to notify the client of <see cref="IMailFolder.UidValidity"/> changes by emitting the
+		/// <see cref="IMailFolder.UidValidityChanged"/> event.</para>
+		/// </remarks>
+		public static readonly ImapEvent AnnotationChange = new ImapEvent ("AnnotationChange", true);
+
+		/// <summary>
+		/// AN IMAP event notification for folders that have been created, deleted, or renamed.
+		/// </summary>
+		/// <remarks>
+		/// <para>These notifications are sent if an affected mailbox name was created, deleted, or renamed.</para>
+		/// <para>As these notifications are received by the client, the apropriate will be emitted:
+		/// <see cref="MailStore.FolderCreated"/>, <see cref="IMailFolder.Deleted"/>, or
+		/// <see cref="IMailFolder.Renamed"/>, respectively.</para>
+		/// <note type="info">If the server supports <see cref="ImapCapabilities.Acl"/>, granting or revocation of the
+		/// <see cref="AccessRight.LookupFolder"/> right to the current user on the affected folder will also be
+		/// considered folder creation or deletion, respectively. If a folder is created or deleted, the folder itself
+		/// and its direct parent (whether it is an existing folder or not) are considered to be affected.</note>
+		/// </remarks>
+		public static readonly ImapEvent MailboxName = new ImapEvent ("MailboxName", false);
+
+		/// <summary>
+		/// An IMAP event notification for folders who have had their subscription status changed.
+		/// </summary>
+		/// <remarks>
+		/// <para>This event requests that the server notifies the client of any subscription changes,
+		/// causing the <see cref="IMailFolder.Subscribed"/> or <see cref="IMailFolder.Unsubscribed"/>
+		/// events to be emitted accordingly on the affected <see cref="IMailFolder"/>.</para>
+		/// </remarks>
+		public static readonly ImapEvent SubscriptionChange = new ImapEvent ("SubscriptionChange", false);
+
+		/// <summary>
+		/// An IMAP event notification for changes to folder metadata.
+		/// </summary>
+		/// <remarks>
+		/// <para>Support for this event type is OPTIONAL unless <see cref="ImapCapabilities.Metadata"/> is supported
+		/// by the server, in which case support for this event type is REQUIRED.</para>
+		/// <para>If the server does support this event, then the <see cref="IMailFolder.MetadataChanged"/> event
+		/// will be emitted whenever metadata changes for any folder included in the <see cref="ImapMailboxFilter"/>.</para>
+		/// </remarks>
+		public static readonly ImapEvent MailboxMetadataChange = new ImapEvent ("MailboxMetadataChange", false);
+
+		/// <summary>
+		/// An IMAP event notification for changes to server metadata.
+		/// </summary>
+		/// <remarks>
+		/// <para>Support for this event type is OPTIONAL unless <see cref="ImapCapabilities.Metadata"/> is supported
+		/// by the server, in which case support for this event type is REQUIRED.</para>
+		/// <para>If the server does support this event, then the <see cref="IMailStore.MetadataChanged"/> event
+		/// will be emitted whenever metadata changes.</para>
+		/// </remarks>
+		public static readonly ImapEvent ServerMetadataChange = new ImapEvent ("ServerMetadataChange", false);
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEvent"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEvent"/> class.
+		/// </remarks>
+		/// <param name="name">The name of the IMAP event.</param>
+		/// <param name="isMessageEvent"><c>true</c> if the event is a message event; otherwise, <c>false</c>.</param>
+		internal ImapEvent (string name, bool isMessageEvent)
+		{
+			IsMessageEvent = isMessageEvent;
+			Name = name;
+		}
+
+		/// <summary>
+		/// Get whether or not this <see cref="T:MailKit.Net.Imap.ImapEvent"/> is a message event.
+		/// </summary>
+		/// <remarks>
+		/// Gets whether or not this <see cref="T:MailKit.Net.Imap.ImapEvent"/> is a message event.
+		/// </remarks>
+		/// <value><c>true</c> if is message event; otherwise, <c>false</c>.</value>
+		internal bool IsMessageEvent {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the name of the IMAP event.
+		/// </summary>
+		/// <remarks>
+		/// Gets the name of the IMAP event.
+		/// </remarks>
+		/// <value>The name of the IMAP event.</value>
+		public string Name {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Format the IMAP NOTIFY command for this particular IMAP mailbox filter.
+		/// </summary>
+		/// <remarks>
+		/// Formats the IMAP NOTIFY command for this particular IMAP mailbox filter.
+		/// </remarks>
+		/// <param name="engine">The IMAP engine.</param>
+		/// <param name="command">The IMAP command builder.</param>
+		/// <param name="args">The IMAP command argument builder.</param>
+		/// <param name="isSelectedFilter"><c>true</c> if the event is being registered for a
+		/// <see cref="ImapMailboxFilter.Selected"/> or <see cref="ImapMailboxFilter.SelectedDelayed"/>
+		/// mailbox filter.</param>
+		internal virtual void Format (ImapEngine engine, StringBuilder command, IList<object> args, bool isSelectedFilter)
+		{
+			command.Append (Name);
+		}
+
+		/// <summary>
+		/// An IMAP event notification for new or appended messages.
+		/// </summary>
+		/// <remarks>
+		/// <para>An IMAP event notification for new or appended messages.</para>
+		/// <para>If the new or appended message is in the selected folder, the folder will emit the
+		/// <see cref="IMailFolder.CountChanged"/> event, followed by a
+		/// <see cref="IMailFolder.MessageSummaryFetched"/> event containing the information requested by the client.</para>
+		/// <note type="note">These events will not be emitted for any message created by the client on this particular folder
+		/// as a result of, for example, a call to
+		/// <see cref="IMailFolder.Append(MimeMessage, MessageFlags, System.Threading.CancellationToken, ITransferProgress)"/>
+		/// or <see cref="IMailFolder.CopyTo(IList{UniqueId}, IMailFolder, System.Threading.CancellationToken)"/>.</note>
+		/// </remarks>
+		public class MessageNew : ImapEvent
+		{
+			readonly MessageSummaryItems items;
+			readonly HashSet<string> headers;
+
+			/// <summary>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEvent.MessageNew"/> class.
+			/// </summary>
+			/// <remarks>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEvent.MessageNew"/> class.
+			/// </remarks>
+			/// <param name="items">The message summary items to automatically retrieve for new messages.</param>
+			public MessageNew (MessageSummaryItems items = MessageSummaryItems.None) : base ("MessageNew", true)
+			{
+				headers = ImapFolder.EmptyHeaderFields;
+				this.items = items;
+			}
+
+			/// <summary>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEvent.MessageNew"/> class.
+			/// </summary>
+			/// <remarks>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEvent.MessageNew"/> class.
+			/// </remarks>
+			/// <param name="items">The message summary items to automatically retrieve for new messages.</param>
+			/// <param name="headers">Additional message headers to retrieve for new messages.</param>
+			public MessageNew (MessageSummaryItems items, HashSet<HeaderId> headers) : this (items)
+			{
+				this.headers = ImapUtils.GetUniqueHeaders (headers);
+			}
+
+			/// <summary>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEvent.MessageNew"/> class.
+			/// </summary>
+			/// <remarks>
+			/// Initializes a new instance of the <see cref="T:MailKit.Net.Imap.ImapEvent.MessageNew"/> class.
+			/// </remarks>
+			/// <param name="items">The message summary items to automatically retrieve for new messages.</param>
+			/// <param name="headers">Additional message headers to retrieve for new messages.</param>
+			public MessageNew (MessageSummaryItems items, HashSet<string> headers) : this (items)
+			{
+				this.headers = ImapUtils.GetUniqueHeaders (headers);
+			}
+
+			/// <summary>
+			/// Format the IMAP NOTIFY command for this particular IMAP mailbox filter.
+			/// </summary>
+			/// <remarks>
+			/// Formats the IMAP NOTIFY command for this particular IMAP mailbox filter.
+			/// </remarks>
+			/// <param name="engine">The IMAP engine.</param>
+			/// <param name="command">The IMAP command builder.</param>
+			/// <param name="args">The IMAP command argument builder.</param>
+			/// <param name="isSelectedFilter"><c>true</c> if the event is being registered for a
+			/// <see cref="ImapMailboxFilter.Selected"/> or <see cref="ImapMailboxFilter.SelectedDelayed"/>
+			/// mailbox filter.</param>
+			internal override void Format (ImapEngine engine, StringBuilder command, IList<object> args, bool isSelectedFilter)
+			{
+				command.Append (Name);
+
+				if (items == MessageSummaryItems.None && headers.Count == 0)
+					return;
+
+				if (!isSelectedFilter)
+					throw new InvalidOperationException ("The MessageNew event cannot have any parameters for mailbox filters other than SELECTED and SELECTED-DELAYED.");
+
+				var xitems = items;
+				bool previewText;
+
+				command.Append (" ");
+				command.Append (ImapFolder.FormatSummaryItems (engine, ref xitems, headers, out previewText, isNotify: true));
+			}
+		}
+	}
+}

src/MailKit/Net/Imap/ImapFolderAnnotations.cs

@@ -0,0 +1,567 @@
+//
+// ImapFolderAnnotations.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+using System.Threading;
+using System.Globalization;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+namespace MailKit.Net.Imap
+{
+	public partial class ImapFolder
+	{
+		async Task<IList<UniqueId>> StoreAsync (IList<UniqueId> uids, ulong? modseq, IList<Annotation> annotations, bool doAsync, CancellationToken cancellationToken)
+		{
+			if (uids == null)
+				throw new ArgumentNullException (nameof (uids));
+
+			if (modseq.HasValue && !supportsModSeq)
+				throw new NotSupportedException ("The ImapFolder does not support mod-sequences.");
+
+			if (annotations == null)
+				throw new ArgumentNullException (nameof (annotations));
+
+			CheckState (true, true);
+
+			if (AnnotationAccess == AnnotationAccess.None)
+				throw new NotSupportedException ("The ImapFolder does not support annotations.");
+
+			if (uids.Count == 0 || annotations.Count == 0)
+				return new UniqueId[0];
+
+			var builder = new StringBuilder ("UID STORE %s ");
+			var values = new List<object> ();
+			UniqueIdSet unmodified = null;
+
+			if (modseq.HasValue)
+				builder.AppendFormat (CultureInfo.InvariantCulture, "(UNCHANGEDSINCE {0}) ", modseq.Value);
+
+			ImapUtils.FormatAnnotations (builder, annotations, values, true);
+			builder.Append ("\r\n");
+
+			var command = builder.ToString ();
+			var args = values.ToArray ();
+
+			foreach (var ic in Engine.QueueCommands (cancellationToken, this, command, uids, args)) {
+				await Engine.RunAsync (ic, doAsync).ConfigureAwait (false);
+
+				ProcessResponseCodes (ic, null);
+
+				if (ic.Response != ImapCommandResponse.Ok)
+					throw ImapCommandException.Create ("STORE", ic);
+
+				ProcessUnmodified (ic, ref unmodified, modseq);
+			}
+
+			if (unmodified == null)
+				return new UniqueId[0];
+
+			return unmodified;
+		}
+
+		/// <summary>
+		/// Store the annotations for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// Stores the annotations for the specified messages.
+		/// </remarks>
+		/// <param name="uids">The UIDs of the messages.</param>
+		/// <param name="annotations">The annotations to store.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="uids"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="annotations"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="uids"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open in read-write mode.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Cannot store annotations without any properties defined.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The <see cref="ImapFolder"/> does not support annotations.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override void Store (IList<UniqueId> uids, IList<Annotation> annotations, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			StoreAsync (uids, null, annotations, false, cancellationToken).GetAwaiter ().GetResult ();
+		}
+
+		/// <summary>
+		/// Asynchronously store the annotations for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously stores the annotations for the specified messages.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="uids">The UIDs of the messages.</param>
+		/// <param name="annotations">The annotations to store.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="uids"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="annotations"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="uids"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open in read-write mode.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Cannot store annotations without any properties defined.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The <see cref="ImapFolder"/> does not support annotations.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override Task StoreAsync (IList<UniqueId> uids, IList<Annotation> annotations, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return StoreAsync (uids, null, annotations, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Store the annotations for the specified messages only if their mod-sequence value is less than the specified value.
+		/// </summary>
+		/// <remarks>
+		/// Stores the annotations for the specified messages only if their mod-sequence value is less than the specified value.
+		/// </remarks>
+		/// <returns>The unique IDs of the messages that were not updated.</returns>
+		/// <param name="uids">The UIDs of the messages.</param>
+		/// <param name="modseq">The mod-sequence value.</param>
+		/// <param name="annotations">The annotations to store.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="uids"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="annotations"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="uids"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open in read-write mode.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Cannot store annotations without any properties defined.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <para>The <see cref="ImapFolder"/> does not support annotations.</para>
+		/// <para>-or-</para>
+		/// <para>The <see cref="ImapFolder"/> does not support mod-sequences.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override IList<UniqueId> Store (IList<UniqueId> uids, ulong modseq, IList<Annotation> annotations, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return StoreAsync (uids, modseq, annotations, false, cancellationToken).GetAwaiter ().GetResult ();
+		}
+
+		/// <summary>
+		/// Asynchronously store the annotations for the specified messages only if their mod-sequence value is less than the specified value.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously stores the annotations for the specified messages only if their mod-sequence value is less than the specified value.
+		/// </remarks>
+		/// <returns>The unique IDs of the messages that were not updated.</returns>
+		/// <param name="uids">The UIDs of the messages.</param>
+		/// <param name="modseq">The mod-sequence value.</param>
+		/// <param name="annotations">The annotations to store.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="uids"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="annotations"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="uids"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open in read-write mode.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Cannot store annotations without any properties defined.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <para>The <see cref="ImapFolder"/> does not support annotations.</para>
+		/// <para>-or-</para>
+		/// <para>The <see cref="ImapFolder"/> does not support mod-sequences.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override Task<IList<UniqueId>> StoreAsync (IList<UniqueId> uids, ulong modseq, IList<Annotation> annotations, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return StoreAsync (uids, modseq, annotations, true, cancellationToken);
+		}
+
+		async Task<IList<int>> StoreAsync (IList<int> indexes, ulong? modseq, IList<Annotation> annotations, bool doAsync, CancellationToken cancellationToken)
+		{
+			if (indexes == null)
+				throw new ArgumentNullException (nameof (indexes));
+
+			if (modseq.HasValue && !supportsModSeq)
+				throw new NotSupportedException ("The ImapFolder does not support mod-sequences.");
+
+			if (annotations == null)
+				throw new ArgumentNullException (nameof (annotations));
+
+			CheckState (true, true);
+
+			if (AnnotationAccess == AnnotationAccess.None)
+				throw new NotSupportedException ("The ImapFolder does not support annotations.");
+
+			if (indexes.Count == 0 || annotations.Count == 0)
+				return new int[0];
+
+			var set = ImapUtils.FormatIndexSet (indexes);
+			var builder = new StringBuilder ("STORE ");
+			var values = new List<object> ();
+
+			builder.AppendFormat ("{0} ", set);
+
+			if (modseq.HasValue)
+				builder.AppendFormat (CultureInfo.InvariantCulture, "(UNCHANGEDSINCE {0}) ", modseq.Value);
+
+			ImapUtils.FormatAnnotations (builder, annotations, values, true);
+			builder.Append ("\r\n");
+
+			var command = builder.ToString ();
+			var args = values.ToArray ();
+
+			var ic = Engine.QueueCommand (cancellationToken, this, command, args);
+
+			await Engine.RunAsync (ic, doAsync).ConfigureAwait (false);
+
+			ProcessResponseCodes (ic, null);
+
+			if (ic.Response != ImapCommandResponse.Ok)
+				throw ImapCommandException.Create ("STORE", ic);
+
+			return GetUnmodified (ic, modseq);
+		}
+
+		/// <summary>
+		/// Store the annotations for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// Stores the annotations for the specified messages.
+		/// </remarks>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="annotations">The annotations to store.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="indexes"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="annotations"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="indexes"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open in read-write mode.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Cannot store annotations without any properties defined.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The <see cref="ImapFolder"/> does not support annotations.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override void Store (IList<int> indexes, IList<Annotation> annotations, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			StoreAsync (indexes, null, annotations, false, cancellationToken).GetAwaiter ().GetResult ();
+		}
+
+		/// <summary>
+		/// Asynchronously store the annotations for the specified messages.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously stores the annotations for the specified messages.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="annotations">The annotations to store.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="indexes"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="annotations"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="indexes"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open in read-write mode.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Cannot store annotations without any properties defined.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The <see cref="ImapFolder"/> does not support annotations.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override Task StoreAsync (IList<int> indexes, IList<Annotation> annotations, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return StoreAsync (indexes, null, annotations, true, cancellationToken);
+		}
+
+		/// <summary>
+		/// Store the annotations for the specified messages only if their mod-sequence value is less than the specified value.
+		/// </summary>
+		/// <remarks>
+		/// Stores the annotations for the specified messages only if their mod-sequence value is less than the specified value.
+		/// </remarks>
+		/// <returns>The indexes of the messages that were not updated.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="modseq">The mod-sequence value.</param>
+		/// <param name="annotations">The annotations to store.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="indexes"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="annotations"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="indexes"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open in read-write mode.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Cannot store annotations without any properties defined.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <para>The <see cref="ImapFolder"/> does not support annotations.</para>
+		/// <para>-or-</para>
+		/// <para>The <see cref="ImapFolder"/> does not support mod-sequences.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override IList<int> Store (IList<int> indexes, ulong modseq, IList<Annotation> annotations, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return StoreAsync (indexes, modseq, annotations, false, cancellationToken).GetAwaiter ().GetResult ();
+		}
+
+		/// <summary>
+		/// Asynchronously store the annotations for the specified messages only if their mod-sequence value is less than the specified value.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously stores the annotations for the specified messages only if their mod-sequence value is less than the specified value.s
+		/// </remarks>
+		/// <returns>The indexes of the messages that were not updated.</returns>
+		/// <param name="indexes">The indexes of the messages.</param>
+		/// <param name="modseq">The mod-sequence value.</param>
+		/// <param name="annotations">The annotations to store.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <para><paramref name="indexes"/> is <c>null</c>.</para>
+		/// <para>-or-</para>
+		/// <para><paramref name="annotations"/> is <c>null</c>.</para>
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// One or more of the <paramref name="indexes"/> is invalid.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="ImapClient"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="ImapClient"/> is not connected.
+		/// </exception>
+		/// <exception cref="ServiceNotAuthenticatedException">
+		/// The <see cref="ImapClient"/> is not authenticated.
+		/// </exception>
+		/// <exception cref="FolderNotOpenException">
+		/// The <see cref="ImapFolder"/> is not currently open in read-write mode.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// Cannot store annotations without any properties defined.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// <para>The <see cref="ImapFolder"/> does not support annotations.</para>
+		/// <para>-or-</para>
+		/// <para>The <see cref="ImapFolder"/> does not support mod-sequences.</para>
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="ImapProtocolException">
+		/// The server's response contained unexpected tokens.
+		/// </exception>
+		/// <exception cref="ImapCommandException">
+		/// The server replied with a NO or BAD response.
+		/// </exception>
+		public override Task<IList<int>> StoreAsync (IList<int> indexes, ulong modseq, IList<Annotation> annotations, CancellationToken cancellationToken = default (CancellationToken))
+		{
+			return StoreAsync (indexes, modseq, annotations, true, cancellationToken);
+		}
+	}
+}

src/MailKit/Net/Imap/ImapFolderConstructorArgs.cs

@@ -0,0 +1,113 @@
+//
+// ImapFolderInfo.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// Constructor arguments for <see cref="ImapFolder"/>.
+	/// </summary>
+	/// <remarks>
+	/// Constructor arguments for <see cref="ImapFolder"/>.
+	/// </remarks>
+	public sealed class ImapFolderConstructorArgs
+	{
+		internal readonly string EncodedName;
+		internal readonly ImapEngine Engine;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapFolderConstructorArgs"/> class.
+		/// </summary>
+		/// <param name="engine">The IMAP command engine.</param>
+		/// <param name="encodedName">The encoded name.</param>
+		/// <param name="attributes">The attributes.</param>
+		/// <param name="delim">The directory separator.</param>
+		internal ImapFolderConstructorArgs (ImapEngine engine, string encodedName, FolderAttributes attributes, char delim) : this ()
+		{
+			FullName = engine.DecodeMailboxName (encodedName);
+			Name = GetBaseName (FullName, delim);
+			DirectorySeparator = delim;
+			EncodedName = encodedName;
+			Attributes = attributes;
+			Engine = engine;
+		}
+
+		ImapFolderConstructorArgs ()
+		{
+		}
+
+		/// <summary>
+		/// Get the folder attributes.
+		/// </summary>
+		/// <remarks>
+		/// Gets the folder attributes.
+		/// </remarks>
+		/// <value>The folder attributes.</value>
+		public FolderAttributes Attributes {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the directory separator.
+		/// </summary>
+		/// <remarks>
+		/// Gets the directory separator.
+		/// </remarks>
+		/// <value>The directory separator.</value>
+		public char DirectorySeparator {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the full name of the folder.
+		/// </summary>
+		/// <remarks>
+		/// This is the equivalent of the full path of a file on a file system.
+		/// </remarks>
+		/// <value>The full name of the folder.</value>
+		public string FullName {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get the name of the folder.
+		/// </summary>
+		/// <remarks>
+		/// This is the equivalent of the file name of a file on the file system.
+		/// </remarks>
+		/// <value>The name of the folder.</value>
+		public string Name {
+			get; private set;
+		}
+
+		static string GetBaseName (string fullName, char delim)
+		{
+			var names = fullName.Split (new [] { delim }, StringSplitOptions.RemoveEmptyEntries);
+
+			return names.Length > 0 ? names[names.Length - 1] : fullName;
+		}
+	}
+}

src/MailKit/Net/Imap/ImapImplementation.cs

@@ -0,0 +1,217 @@
+//
+// ImapImplementation.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Collections.Generic;
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// The details of an IMAP client or server implementation.
+	/// </summary>
+	/// <remarks>
+	/// Allows an IMAP client and server to share their implementation details
+	/// with each other for the purposes of debugging.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+	/// </example>
+	public class ImapImplementation
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapImplementation"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="MailKit.Net.Imap.ImapImplementation"/>.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		public ImapImplementation ()
+		{
+			Properties = new Dictionary<string, string> ();
+		}
+
+		string GetProperty (string property)
+		{
+			string value;
+
+			Properties.TryGetValue (property, out value);
+
+			return value;
+		}
+
+		/// <summary>
+		/// Get the identification properties.
+		/// </summary>
+		/// <remarks>
+		/// Gets the dictionary of raw identification properties.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The properties.</value>
+		public Dictionary<string, string> Properties {
+			get; private set;
+		}
+
+		/// <summary>
+		/// Get or set the name of the program.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the name of the program.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The program name.</value>
+		public string Name {
+			get { return GetProperty ("name"); }
+			set { Properties["name"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the version of the program.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the version of the program.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\ImapExamples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The program version.</value>
+		public string Version {
+			get { return GetProperty ("version"); }
+			set { Properties["version"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the name of the operating system.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the name of the operating system.
+		/// </remarks>
+		/// <value>The name of the operation system.</value>
+		public string OS {
+			get { return GetProperty ("os"); }
+			set { Properties["os"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the version of the operating system.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the version of the operating system.
+		/// </remarks>
+		/// <value>The version of the operation system.</value>
+		public string OSVersion {
+			get { return GetProperty ("os-version"); }
+			set { Properties["os-version"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the name of the vendor.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the name of the vendor.
+		/// </remarks>
+		/// <value>The name of the vendor.</value>
+		public string Vendor {
+			get { return GetProperty ("vendor"); }
+			set { Properties["vendor"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the support URL.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the support URL.
+		/// </remarks>
+		/// <value>The support URL.</value>
+		public string SupportUrl {
+			get { return GetProperty ("support-url"); }
+			set { Properties["support-url"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the postal address of the vendor.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the postal address of the vendor.
+		/// </remarks>
+		/// <value>The postal address.</value>
+		public string Address {
+			get { return GetProperty ("address"); }
+			set { Properties["address"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the release date of the program.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the release date of the program.
+		/// </remarks>
+		/// <value>The release date.</value>
+		public string ReleaseDate {
+			get { return GetProperty ("date"); }
+			set { Properties["date"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the command used to start the program.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the command used to start the program.
+		/// </remarks>
+		/// <value>The command used to start the program.</value>
+		public string Command {
+			get { return GetProperty ("command"); }
+			set { Properties["command"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the command-line arguments used to start the program.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets the command-line arguments used to start the program.
+		/// </remarks>
+		/// <value>The command-line arguments used to start the program.</value>
+		public string Arguments {
+			get { return GetProperty ("arguments"); }
+			set { Properties["arguments"] = value; }
+		}
+
+		/// <summary>
+		/// Get or set the environment variables available to the program.
+		/// </summary>
+		/// <remarks>
+		/// Get or set the environment variables available to the program.
+		/// </remarks>
+		/// <value>The environment variables.</value>
+		public string Environment {
+			get { return GetProperty ("environment"); }
+			set { Properties["environment"] = value; }
+		}
+	}
+}

src/MailKit/Net/Imap/ImapProtocolException.cs

@@ -0,0 +1,109 @@
+//
+// ImapException.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+#if SERIALIZABLE
+using System.Security;
+using System.Runtime.Serialization;
+#endif
+
+namespace MailKit.Net.Imap {
+	/// <summary>
+	/// An IMAP protocol exception.
+	/// </summary>
+	/// <remarks>
+	/// The exception that is thrown when there is an error communicating with an IMAP server. An
+	/// <see cref="ImapProtocolException"/> is typically fatal and requires the <see cref="ImapClient"/>
+	/// to be reconnected.
+	/// </remarks>
+#if SERIALIZABLE
+	[Serializable]
+#endif
+	public class ImapProtocolException : ProtocolException
+	{
+#if SERIALIZABLE
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapProtocolException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ImapProtocolException"/> from the serialized data.
+		/// </remarks>
+		/// <param name="info">The serialization info.</param>
+		/// <param name="context">The streaming context.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="info"/> is <c>null</c>.
+		/// </exception>
+		[SecuritySafeCritical]
+		protected ImapProtocolException (SerializationInfo info, StreamingContext context) : base (info, context)
+		{
+		}
+#endif
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapProtocolException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ImapProtocolException"/>.
+		/// </remarks>
+		/// <param name="message">The error message.</param>
+		/// <param name="innerException">An inner exception.</param>
+		public ImapProtocolException (string message, Exception innerException) : base (message, innerException)
+		{
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapProtocolException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ImapProtocolException"/>.
+		/// </remarks>
+		/// <param name="message">The error message.</param>
+		public ImapProtocolException (string message) : base (message)
+		{
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="MailKit.Net.Imap.ImapProtocolException"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Creates a new <see cref="ImapProtocolException"/>.
+		/// </remarks>
+		public ImapProtocolException ()
+		{
+		}
+
+		/// <summary>
+		/// Gets or sets whether or not this exception was thrown due to an unexpected token.
+		/// </summary>
+		/// <remarks>
+		/// Gets or sets whether or not this exception was thrown due to an unexpected token.
+		/// </remarks>
+		/// <value><c>true</c> if an unexpected token was encountered; otherwise, <c>false</c>.</value>
+		internal bool UnexpectedToken {
+			get; set;
+		}
+	}
+}

src/MailKit/Net/Imap/ImapResponseCode.cs

@@ -0,0 +1,373 @@
+//
+// ImapResponseCode.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+namespace MailKit.Net.Imap {
+	enum ImapResponseCodeType : byte {
+		Alert,
+		BadCharset,
+		Capability,
+		NewName,
+		Parse,
+		PermanentFlags,
+		ReadOnly,
+		ReadWrite,
+		TryCreate,
+		UidNext,
+		UidValidity,
+		Unseen,
+
+		// RESP-CODES introduced in rfc2221:
+		Referral,
+
+		// RESP-CODES introduced in rfc3516,
+		UnknownCte,
+
+		// RESP-CODES introduced in rfc4315:
+		AppendUid,
+		CopyUid,
+		UidNotSticky,
+
+		// RESP-CODES introduced in rfc4467:
+		UrlMech,
+
+		// RESP-CODES introduced in rfc4469:
+		BadUrl,
+		TooBig,
+
+		// RESP-CODES introduced in rfc4551:
+		HighestModSeq,
+		Modified,
+		NoModSeq,
+
+		// RESP-CODES introduced in rfc4978:
+		CompressionActive,
+
+		// RESP-CODES introduced in rfc5162:
+		Closed,
+
+		// RESP-CODES introduced in rfc5182:
+		NotSaved,
+
+		// RESP-CODES introduced in rfc5255:
+		BadComparator,
+
+		// RESP-CODES introduced in rfc5257:
+		Annotate,
+		Annotations,
+
+		// RESP-CODES introduced in rfc5259:
+		MaxConvertMessages,
+		MaxConvertParts,
+		TempFail,
+
+		// RESP-CODES introduced in rfc5267:
+		NoUpdate,
+
+		// RESP-CODES introduced in rfc5464:
+		Metadata,
+
+		// RESP-CODES introduced in rfc5465:
+		NotificationOverflow,
+		BadEvent,
+
+		// RESP-CODES introduced in rfc5466:
+		UndefinedFilter,
+
+		// RESP-CODES introduced in rfc5530:
+		Unavailable,
+		AuthenticationFailed,
+		AuthorizationFailed,
+		Expired,
+		PrivacyRequired,
+		ContactAdmin,
+		NoPerm,
+		InUse,
+		ExpungeIssued,
+		Corruption,
+		ServerBug,
+		ClientBug,
+		CanNot,
+		Limit,
+		OverQuota,
+		AlreadyExists,
+		NonExistent,
+
+		// RESP-CODES introduced in rfc6154:
+		UseAttr,
+
+		// RESP-CODES introduced in rfc8474:
+		MailboxId,
+
+		Unknown       = 255
+	}
+
+	class ImapResponseCode
+	{
+		public readonly ImapResponseCodeType Type;
+		public bool IsTagged, IsError;
+		public string Message;
+
+		internal ImapResponseCode (ImapResponseCodeType type, bool isError)
+		{
+			IsError = isError;
+			Type = type;
+		}
+
+		public static ImapResponseCode Create (ImapResponseCodeType type)
+		{
+			switch (type) {
+			case ImapResponseCodeType.Alert:                return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.BadCharset:           return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.Capability:           return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.NewName:              return new NewNameResponseCode (type);
+			case ImapResponseCodeType.Parse:                return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.PermanentFlags:       return new PermanentFlagsResponseCode (type);
+			case ImapResponseCodeType.ReadOnly:             return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.ReadWrite:            return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.TryCreate:            return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.UidNext:              return new UidNextResponseCode (type);
+			case ImapResponseCodeType.UidValidity:          return new UidValidityResponseCode (type);
+			case ImapResponseCodeType.Unseen:               return new UnseenResponseCode (type);
+			case ImapResponseCodeType.Referral:             return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.UnknownCte:           return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.AppendUid:            return new AppendUidResponseCode (type);
+			case ImapResponseCodeType.CopyUid:              return new CopyUidResponseCode (type);
+			case ImapResponseCodeType.UidNotSticky:         return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.UrlMech:              return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.BadUrl:               return new BadUrlResponseCode (type);
+			case ImapResponseCodeType.TooBig:               return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.HighestModSeq:        return new HighestModSeqResponseCode (type);
+			case ImapResponseCodeType.Modified:             return new ModifiedResponseCode (type);
+			case ImapResponseCodeType.NoModSeq:             return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.CompressionActive:    return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.Closed:               return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.NotSaved:             return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.BadComparator:        return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.Annotate:             return new AnnotateResponseCode (type);
+			case ImapResponseCodeType.Annotations:          return new AnnotationsResponseCode (type);
+			case ImapResponseCodeType.MaxConvertMessages:   return new MaxConvertResponseCode (type);
+			case ImapResponseCodeType.MaxConvertParts:      return new MaxConvertResponseCode (type);
+			case ImapResponseCodeType.TempFail:             return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.NoUpdate:             return new NoUpdateResponseCode (type);
+			case ImapResponseCodeType.Metadata:             return new MetadataResponseCode (type);
+			case ImapResponseCodeType.NotificationOverflow: return new ImapResponseCode (type, false);
+			case ImapResponseCodeType.BadEvent:             return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.UndefinedFilter:      return new UndefinedFilterResponseCode (type);
+			case ImapResponseCodeType.Unavailable:          return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.AuthenticationFailed: return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.AuthorizationFailed:  return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.Expired:              return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.PrivacyRequired:      return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.ContactAdmin:         return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.NoPerm:               return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.InUse:                return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.ExpungeIssued:        return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.Corruption:           return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.ServerBug:            return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.ClientBug:            return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.CanNot:               return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.Limit:                return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.OverQuota:            return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.AlreadyExists:        return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.NonExistent:          return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.UseAttr:              return new ImapResponseCode (type, true);
+			case ImapResponseCodeType.MailboxId:            return new MailboxIdResponseCode (type);
+			default:                                        return new ImapResponseCode (type, true);
+			}
+		}
+	}
+
+	class NewNameResponseCode : ImapResponseCode
+	{
+		public string OldName;
+		public string NewName;
+
+		internal NewNameResponseCode (ImapResponseCodeType type) : base (type, false)
+		{
+		}
+	}
+
+	class PermanentFlagsResponseCode : ImapResponseCode
+	{
+		public MessageFlags Flags;
+
+		internal PermanentFlagsResponseCode (ImapResponseCodeType type) : base (type, false)
+		{
+		}
+	}
+
+	class UidNextResponseCode : ImapResponseCode
+	{
+		public UniqueId Uid;
+
+		internal UidNextResponseCode (ImapResponseCodeType type) : base (type, false)
+		{
+		}
+	}
+
+	class UidValidityResponseCode : ImapResponseCode
+	{
+		public uint UidValidity;
+
+		internal UidValidityResponseCode (ImapResponseCodeType type) : base (type, false)
+		{
+		}
+	}
+
+	class UnseenResponseCode : ImapResponseCode
+	{
+		public int Index;
+
+		internal UnseenResponseCode (ImapResponseCodeType type) : base (type, false)
+		{
+		}
+	}
+
+	class AppendUidResponseCode : UidValidityResponseCode
+	{
+		public UniqueIdSet UidSet;
+
+		internal AppendUidResponseCode (ImapResponseCodeType type) : base (type)
+		{
+		}
+	}
+
+	class CopyUidResponseCode : UidValidityResponseCode
+	{
+		public UniqueIdSet SrcUidSet, DestUidSet;
+
+		internal CopyUidResponseCode (ImapResponseCodeType type) : base (type)
+		{
+		}
+	}
+
+	class BadUrlResponseCode : ImapResponseCode
+	{
+		public string BadUrl;
+
+		internal BadUrlResponseCode (ImapResponseCodeType type) : base (type, true)
+		{
+		}
+	}
+
+	class HighestModSeqResponseCode : ImapResponseCode
+	{
+		public ulong HighestModSeq;
+
+		internal HighestModSeqResponseCode (ImapResponseCodeType type) : base (type, false)
+		{
+		}
+	}
+
+	class ModifiedResponseCode : ImapResponseCode
+	{
+		public UniqueIdSet UidSet;
+
+		internal ModifiedResponseCode (ImapResponseCodeType type) : base (type, false)
+		{
+		}
+	}
+
+	class MaxConvertResponseCode : ImapResponseCode
+	{
+		public uint MaxConvert;
+
+		internal MaxConvertResponseCode (ImapResponseCodeType type) : base (type, true)
+		{
+		}
+	}
+
+	class NoUpdateResponseCode : ImapResponseCode
+	{
+		public string Tag;
+
+		internal NoUpdateResponseCode (ImapResponseCodeType type) : base (type, true)
+		{
+		}
+	}
+
+	enum AnnotateResponseCodeSubType
+	{
+		TooBig,
+		TooMany
+	}
+
+	class AnnotateResponseCode : ImapResponseCode
+	{
+		public AnnotateResponseCodeSubType SubType;
+
+		internal AnnotateResponseCode (ImapResponseCodeType type) : base (type, true)
+		{
+		}
+	}
+
+	class AnnotationsResponseCode : ImapResponseCode
+	{
+		public AnnotationAccess Access;
+		public AnnotationScope Scopes;
+		public uint MaxSize;
+
+		internal AnnotationsResponseCode (ImapResponseCodeType type) : base (type, false)
+		{
+		}
+	}
+
+	enum MetadataResponseCodeSubType
+	{
+		LongEntries,
+		MaxSize,
+		TooMany,
+		NoPrivate
+	}
+
+	class MetadataResponseCode : ImapResponseCode
+	{
+		public MetadataResponseCodeSubType SubType;
+		public uint Value;
+
+		internal MetadataResponseCode (ImapResponseCodeType type) : base (type, true)
+		{
+		}
+	}
+
+	class UndefinedFilterResponseCode : ImapResponseCode
+	{
+		public string Name;
+
+		internal UndefinedFilterResponseCode (ImapResponseCodeType type) : base (type, true)
+		{
+		}
+	}
+
+	class MailboxIdResponseCode : ImapResponseCode
+	{
+		public string MailboxId;
+
+		internal MailboxIdResponseCode (ImapResponseCodeType type) : base (type, false)
+		{
+		}
+	}
+}

src/MailKit/Net/Imap/ImapSearchQueryOptimizer.cs

@@ -0,0 +1,83 @@
+//
+// ImapSearchQueryOptimizer.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using MailKit.Search;
+
+namespace MailKit.Net.Imap {
+	class ImapSearchQueryOptimizer : ISearchQueryOptimizer
+	{
+		#region ISearchQueryOptimizer implementation
+
+		public SearchQuery Reduce (SearchQuery expr)
+		{
+			if (expr.Term == SearchTerm.And) {
+				var and = (BinarySearchQuery) expr;
+
+				if (and.Left.Term == SearchTerm.All)
+					return and.Right.Optimize (this);
+
+				if (and.Right.Term == SearchTerm.All)
+					return and.Left.Optimize (this);
+			} else if (expr.Term == SearchTerm.Or) {
+				var or = (BinarySearchQuery) expr;
+
+				if (or.Left.Term == SearchTerm.All)
+					return SearchQuery.All;
+
+				if (or.Right.Term == SearchTerm.All)
+					return SearchQuery.All;
+			} else if (expr.Term == SearchTerm.Not) {
+				var unary = (UnarySearchQuery) expr;
+
+				switch (unary.Operand.Term) {
+				case SearchTerm.Not: return ((UnarySearchQuery) unary.Operand).Operand.Optimize (this);
+				case SearchTerm.NotAnswered: return SearchQuery.Answered;
+				case SearchTerm.Answered: return SearchQuery.NotAnswered;
+				case SearchTerm.NotDeleted: return SearchQuery.Deleted;
+				case SearchTerm.Deleted: return SearchQuery.NotDeleted;
+				case SearchTerm.NotDraft: return SearchQuery.Draft;
+				case SearchTerm.Draft: return SearchQuery.NotDraft;
+				case SearchTerm.NotFlagged: return SearchQuery.Flagged;
+				case SearchTerm.Flagged: return SearchQuery.NotFlagged;
+				case SearchTerm.NotRecent: return SearchQuery.Recent;
+				case SearchTerm.Recent: return SearchQuery.NotRecent;
+				case SearchTerm.NotSeen: return SearchQuery.Seen;
+				case SearchTerm.Seen: return SearchQuery.NotSeen;
+				}
+
+				if (unary.Operand.Term == SearchTerm.Keyword)
+					return new TextSearchQuery (SearchTerm.NotKeyword, ((TextSearchQuery) unary.Operand).Text);
+
+				if (unary.Operand.Term == SearchTerm.NotKeyword)
+					return new TextSearchQuery (SearchTerm.Keyword, ((TextSearchQuery) unary.Operand).Text);
+			}
+
+			return expr;
+		}
+
+		#endregion
+	}
+}

src/MailKit/Net/Imap/ImapToken.cs

@@ -0,0 +1,80 @@
+//
+// ImapToken.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Globalization;
+
+namespace MailKit.Net.Imap {
+	enum ImapTokenType {
+		NoData        = -7,
+		Error         = -6,
+		Nil           = -5,
+		Atom          = -4,
+		Flag          = -3,
+		QString       = -2,
+		Literal       = -1,
+
+		// character tokens:
+		Eoln          = (int) '\n',
+		OpenParen     = (int) '(',
+		CloseParen    = (int) ')',
+		Asterisk      = (int) '*',
+		OpenBracket   = (int) '[',
+		CloseBracket  = (int) ']',
+	}
+
+	class ImapToken
+	{
+		public readonly ImapTokenType Type;
+		public readonly object Value;
+
+		public ImapToken (ImapTokenType type, object value = null)
+		{
+			Value = value;
+			Type = type;
+
+			//System.Console.WriteLine ("token: {0}", this);
+		}
+
+		public override string ToString ()
+		{
+			switch (Type) {
+			case ImapTokenType.NoData:       return "<no data>";
+			case ImapTokenType.Nil:          return "NIL";
+			case ImapTokenType.Atom:         return "[atom: " + (string) Value + "]";
+			case ImapTokenType.Flag:         return "[flag: " + (string) Value + "]";
+			case ImapTokenType.QString:      return "[qstring: \"" + (string) Value + "\"]";
+			case ImapTokenType.Literal:      return "{" + (int) Value + "}";
+			case ImapTokenType.Eoln:         return "'\\n'";
+			case ImapTokenType.OpenParen:    return "'('";
+			case ImapTokenType.CloseParen:   return "')'";
+			case ImapTokenType.Asterisk:     return "'*'";
+			case ImapTokenType.OpenBracket:  return "'['";
+			case ImapTokenType.CloseBracket: return "']'";
+			default:                         return string.Format (CultureInfo.InvariantCulture, "[{0}: '{1}']", Type, Value);
+			}
+		}
+	}
+}

src/MailKit/Net/NetworkStream.cs

@@ -0,0 +1,298 @@
+//
+// NetworkStream.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.IO;
+using System.Threading;
+using System.Net.Sockets;
+using System.Threading.Tasks;
+
+namespace MailKit.Net
+{
+	class NetworkStream : Stream
+	{
+		SocketAsyncEventArgs send;
+		SocketAsyncEventArgs recv;
+		bool ownsSocket;
+		bool connected;
+
+		public NetworkStream (Socket socket, bool ownsSocket)
+		{
+			send = new SocketAsyncEventArgs ();
+			send.Completed += AsyncOperationCompleted;
+			send.AcceptSocket = socket;
+
+			recv = new SocketAsyncEventArgs ();
+			recv.Completed += AsyncOperationCompleted;
+			recv.AcceptSocket = socket;
+
+			this.ownsSocket = ownsSocket;
+			connected = socket.Connected;
+			Socket = socket;
+		}
+
+		~NetworkStream ()
+		{
+			Dispose (false);
+		}
+
+		public Socket Socket {
+			get; private set;
+		}
+
+		public bool DataAvailable {
+			get { return connected && Socket.Available > 0; }
+		}
+
+		public override bool CanRead {
+			get { return connected; }
+		}
+
+		public override bool CanWrite {
+			get { return connected; }
+		}
+
+		public override bool CanSeek {
+			get { return false; }
+		}
+
+		public override bool CanTimeout {
+			get { return connected; }
+		}
+
+		public override long Length {
+			get { throw new NotSupportedException (); }
+		}
+
+		public override long Position {
+			get { throw new NotSupportedException (); }
+			set { throw new NotSupportedException (); }
+		}
+
+		public override int ReadTimeout {
+			get {
+				int timeout = Socket.ReceiveTimeout;
+
+				return timeout == 0 ? Timeout.Infinite : timeout;
+			}
+			set {
+				if (value <= 0 && value != Timeout.Infinite)
+					throw new ArgumentOutOfRangeException (nameof (value));
+
+				Socket.ReceiveTimeout = value;
+			}
+		}
+
+		public override int WriteTimeout {
+			get {
+				int timeout = Socket.SendTimeout;
+
+				return timeout == 0 ? Timeout.Infinite : timeout;
+			}
+			set {
+				if (value <= 0 && value != Timeout.Infinite)
+					throw new ArgumentOutOfRangeException (nameof (value));
+
+				Socket.SendTimeout = value;
+			}
+		}
+
+		void AsyncOperationCompleted (object sender, SocketAsyncEventArgs args)
+		{
+			var tcs = (TaskCompletionSource<bool>) args.UserToken;
+
+			if (args.SocketError == SocketError.Success) {
+				tcs.TrySetResult (true);
+				return;
+			}
+
+			tcs.TrySetException (new SocketException ((int) args.SocketError));
+		}
+
+		void Disconnect ()
+		{
+			try {
+				Socket.Dispose ();
+			} catch {
+				return;
+			} finally {
+				connected = false;
+				send.Dispose ();
+				send = null;
+				recv.Dispose ();
+				recv = null;
+			}
+		}
+
+		public override int Read (byte[] buffer, int offset, int count)
+		{
+			try {
+				return Socket.Receive (buffer, offset, count, SocketFlags.None);
+			} catch (SocketException ex) {
+				throw new IOException (ex.Message, ex);
+			}
+		}
+
+		public override async Task<int> ReadAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+		{
+			cancellationToken.ThrowIfCancellationRequested ();
+
+			var tcs = new TaskCompletionSource<bool> ();
+
+			using (var timeout = new CancellationTokenSource (ReadTimeout)) {
+				using (var linked = CancellationTokenSource.CreateLinkedTokenSource (cancellationToken, timeout.Token)) {
+					using (var registration = linked.Token.Register (() => tcs.TrySetCanceled (), false)) {
+						recv.SetBuffer (buffer, offset, count);
+						recv.UserToken = tcs;
+
+						if (!Socket.ReceiveAsync (recv))
+							AsyncOperationCompleted (null, recv);
+
+						try {
+							await tcs.Task.ConfigureAwait (false);
+							return recv.BytesTransferred;
+						} catch (OperationCanceledException) {
+							if (Socket.Connected)
+								Socket.Shutdown (SocketShutdown.Both);
+
+							Disconnect ();
+							throw;
+						} catch (Exception ex) {
+							Disconnect ();
+							if (ex is SocketException)
+								throw new IOException (ex.Message, ex);
+							throw;
+						}
+					}
+				}
+			}
+		}
+
+		public override void Write (byte[] buffer, int offset, int count)
+		{
+			try {
+				Socket.Send (buffer, offset, count, SocketFlags.None);
+			} catch (SocketException ex) {
+				throw new IOException (ex.Message, ex);
+			}
+		}
+
+		public override async Task WriteAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+		{
+			cancellationToken.ThrowIfCancellationRequested ();
+
+			var tcs = new TaskCompletionSource<bool> ();
+
+			using (var timeout = new CancellationTokenSource (WriteTimeout)) {
+				using (var linked = CancellationTokenSource.CreateLinkedTokenSource (cancellationToken, timeout.Token)) {
+					using (var registration = linked.Token.Register (() => tcs.TrySetCanceled (), false)) {
+						send.SetBuffer (buffer, offset, count);
+						send.UserToken = tcs;
+
+						if (!Socket.SendAsync (send))
+							AsyncOperationCompleted (null, send);
+
+						try {
+							await tcs.Task.ConfigureAwait (false);
+						} catch (OperationCanceledException) {
+							if (Socket.Connected)
+								Socket.Shutdown (SocketShutdown.Both);
+
+							Disconnect ();
+							throw;
+						} catch (Exception ex) {
+							Disconnect ();
+							if (ex is SocketException)
+								throw new IOException (ex.Message, ex);
+							throw;
+						}
+					}
+				}
+			}
+		}
+
+		public override void Flush ()
+		{
+		}
+
+		public override Task FlushAsync (CancellationToken cancellationToken)
+		{
+			return Task.FromResult (true);
+		}
+
+		public override long Seek (long offset, SeekOrigin origin)
+		{
+			throw new NotSupportedException ();
+		}
+
+		public override void SetLength (long value)
+		{
+			throw new NotSupportedException ();
+		}
+
+		public static NetworkStream Get (Stream stream)
+		{
+			if (stream is CompressedStream compressed)
+				stream = compressed.InnerStream;
+
+			if (stream is SslStream ssl)
+				stream = ssl.InnerStream;
+
+			return stream as NetworkStream;
+		}
+
+		public void Poll (SelectMode mode, CancellationToken cancellationToken)
+		{
+			if (!cancellationToken.CanBeCanceled)
+				return;
+
+			do {
+				cancellationToken.ThrowIfCancellationRequested ();
+				// wait 1/4 second and then re-check for cancellation
+			} while (!Socket.Poll (250000, mode));
+
+			cancellationToken.ThrowIfCancellationRequested ();
+		}
+
+		protected override void Dispose (bool disposing)
+		{
+			if (disposing) {
+				if (ownsSocket && connected) {
+					ownsSocket = false;
+					Disconnect ();
+				} else {
+					send?.Dispose ();
+					send = null;
+
+					recv?.Dispose ();
+					recv = null;
+				}
+			}
+
+			base.Dispose (disposing);
+		}
+	}
+}

src/MailKit/Net/Pop3/IPop3Client.cs

@@ -0,0 +1,309 @@
+//
+// IPop3Client.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Threading;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+namespace MailKit.Net.Pop3 {
+	/// <summary>
+	/// An interface for a POP3 client.
+	/// </summary>
+	/// <remarks>
+	/// Implemented by <see cref="MailKit.Net.Pop3.Pop3Client"/>.
+	/// </remarks>
+	public interface IPop3Client : IMailSpool
+	{
+		/// <summary>
+		/// Gets the capabilities supported by the POP3 server.
+		/// </summary>
+		/// <remarks>
+		/// The capabilities will not be known until a successful connection has been made 
+		/// and may change once the client is authenticated.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\Pop3Examples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The capabilities.</value>
+		/// <exception cref="System.ArgumentException">
+		/// Capabilities cannot be enabled, they may only be disabled.
+		/// </exception>
+		Pop3Capabilities Capabilities { get; set; }
+
+		/// <summary>
+		/// Gets the expiration policy.
+		/// </summary>
+		/// <remarks>
+		/// <para>If the server supports the EXPIRE capability (<see cref="Pop3Capabilities.Expire"/>), the value
+		/// of the <see cref="ExpirePolicy"/> property will reflect the value advertized by the server.</para>
+		/// <para>A value of <c>-1</c> indicates that messages will never expire.</para>
+		/// <para>A value of <c>0</c> indicates that messages that have been retrieved during the current session
+		/// will be purged immediately after the connection is closed via the <c>QUIT</c> command.</para>
+		/// <para>Values larger than <c>0</c> indicate the minimum number of days that the server will retain
+		/// messages which have been retrieved.</para>
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\Pop3Examples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The expiration policy.</value>
+		int ExpirePolicy { get; }
+
+		/// <summary>
+		/// Gets the implementation details of the server.
+		/// </summary>
+		/// <remarks>
+		/// If the server advertizes its implementation details, this value will be set to a string containing the
+		/// information details provided by the server.
+		/// </remarks>
+		/// <value>The implementation details.</value>
+		string Implementation { get; }
+
+		/// <summary>
+		/// Gets the minimum delay, in milliseconds, between logins.
+		/// </summary>
+		/// <remarks>
+		/// If the server supports the LOGIN-DELAY capability (<see cref="Pop3Capabilities.LoginDelay"/>), this value
+		/// will be set to the minimum number of milliseconds that the client must wait between logins.
+		/// </remarks>
+		/// <example>
+		/// <code language="c#" source="Examples\Pop3Examples.cs" region="Capabilities"/>
+		/// </example>
+		/// <value>The login delay.</value>
+		int LoginDelay { get; }
+
+		/// <summary>
+		/// Enable UTF8 mode.
+		/// </summary>
+		/// <remarks>
+		/// The POP3 UTF8 extension allows the client to retrieve messages in the UTF-8 encoding and
+		/// may also allow the user to authenticate using a UTF-8 encoded username or password.
+		/// </remarks>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="Pop3Client"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="Pop3Client"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// The <see cref="Pop3Client"/> has already been authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The POP3 server does not support the UTF8 extension.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="Pop3CommandException">
+		/// The POP3 command failed.
+		/// </exception>
+		/// <exception cref="Pop3ProtocolException">
+		/// A POP3 protocol error occurred.
+		/// </exception>
+		void EnableUTF8 (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously enable UTF8 mode.
+		/// </summary>
+		/// <remarks>
+		/// The POP3 UTF8 extension allows the client to retrieve messages in the UTF-8 encoding and
+		/// may also allow the user to authenticate using a UTF-8 encoded username or password.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="Pop3Client"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="Pop3Client"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.InvalidOperationException">
+		/// The <see cref="Pop3Client"/> has already been authenticated.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The POP3 server does not support the UTF8 extension.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="Pop3CommandException">
+		/// The POP3 command failed.
+		/// </exception>
+		/// <exception cref="Pop3ProtocolException">
+		/// A POP3 protocol error occurred.
+		/// </exception>
+		Task EnableUTF8Async (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Get the list of languages supported by the POP3 server.
+		/// </summary>
+		/// <remarks>
+		/// If the POP3 server supports the LANG extension, it is possible to
+		/// query the list of languages supported by the POP3 server that can
+		/// be used for error messages.
+		/// </remarks>
+		/// <returns>The supported languages.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="Pop3Client"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="Pop3Client"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The POP3 server does not support the LANG extension.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="Pop3CommandException">
+		/// The POP3 command failed.
+		/// </exception>
+		/// <exception cref="Pop3ProtocolException">
+		/// A POP3 protocol error occurred.
+		/// </exception>
+		IList<Pop3Language> GetLanguages (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously get the list of languages supported by the POP3 server.
+		/// </summary>
+		/// <remarks>
+		/// If the POP3 server supports the LANG extension, it is possible to
+		/// query the list of languages supported by the POP3 server that can
+		/// be used for error messages.
+		/// </remarks>
+		/// <returns>The supported languages.</returns>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="Pop3Client"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="Pop3Client"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The POP3 server does not support the LANG extension.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="Pop3CommandException">
+		/// The POP3 command failed.
+		/// </exception>
+		/// <exception cref="Pop3ProtocolException">
+		/// A POP3 protocol error occurred.
+		/// </exception>
+		Task<IList<Pop3Language>> GetLanguagesAsync (CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Set the language used by the POP3 server for error messages.
+		/// </summary>
+		/// <remarks>
+		/// If the POP3 server supports the LANG extension, it is possible to
+		/// set the language used by the POP3 server for error messages.
+		/// </remarks>
+		/// <param name="lang">The language code.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="lang"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="lang"/> is empty.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="Pop3Client"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="Pop3Client"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The POP3 server does not support the LANG extension.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="Pop3CommandException">
+		/// The POP3 command failed.
+		/// </exception>
+		/// <exception cref="Pop3ProtocolException">
+		/// A POP3 protocol error occurred.
+		/// </exception>
+		void SetLanguage (string lang, CancellationToken cancellationToken = default (CancellationToken));
+
+		/// <summary>
+		/// Asynchronously set the language used by the POP3 server for error messages.
+		/// </summary>
+		/// <remarks>
+		/// If the POP3 server supports the LANG extension, it is possible to
+		/// set the language used by the POP3 server for error messages.
+		/// </remarks>
+		/// <returns>An asynchronous task context.</returns>
+		/// <param name="lang">The language code.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="lang"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// <paramref name="lang"/> is empty.
+		/// </exception>
+		/// <exception cref="System.ObjectDisposedException">
+		/// The <see cref="Pop3Client"/> has been disposed.
+		/// </exception>
+		/// <exception cref="ServiceNotConnectedException">
+		/// The <see cref="Pop3Client"/> is not connected.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.NotSupportedException">
+		/// The POP3 server does not support the LANG extension.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		/// <exception cref="Pop3CommandException">
+		/// The POP3 command failed.
+		/// </exception>
+		/// <exception cref="Pop3ProtocolException">
+		/// A POP3 protocol error occurred.
+		/// </exception>
+		Task SetLanguageAsync (string lang, CancellationToken cancellationToken = default (CancellationToken));
+	}
+}

src/MailKit/Net/Pop3/Pop3Capabilities.cs

@@ -0,0 +1,129 @@
+//
+// Pop3Capabilities.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+
+namespace MailKit.Net.Pop3 {
+	/// <summary>
+	/// Capabilities supported by a POP3 server.
+	/// </summary>
+	/// <remarks>
+	/// Capabilities are read as part of the response to the <c>CAPA</c> command that
+	/// is issued during the connection and authentication phases of the
+	/// <see cref="Pop3Client"/>.
+	/// </remarks>
+	/// <example>
+	/// <code language="c#" source="Examples\Pop3Examples.cs" region="Capabilities"/>
+	/// </example>
+	[Flags]
+	public enum Pop3Capabilities : uint {
+		/// <summary>
+		/// The server does not support any additional extensions.
+		/// </summary>
+		None                   = 0,
+
+		/// <summary>
+		/// The server supports <a href="https://tools.ietf.org/html/rfc1939#page-15">APOP</a>
+		/// authentication.
+		/// </summary>
+		Apop                   = 1 << 0,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2449">EXPIRE</a> extension
+		/// and defines the expiration policy for messages (see <see cref="Pop3Client.ExpirePolicy"/>).
+		/// </summary>
+		Expire                 = 1 << 1,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2449">LOGIN-DELAY</a> extension,
+		/// allowing the server to specify to the client a minimum number of seconds between login attempts
+		/// (see <see cref="Pop3Client.LoginDelay"/>).
+		/// </summary>
+		LoginDelay             = 1 << 2,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2449">PIPELINING</a> extension,
+		/// allowing the client to batch multiple requests to the server at at time.
+		/// </summary>
+		Pipelining             = 1 << 3,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2449">RESP-CODES</a> extension,
+		/// allowing the server to provide clients with extended information in error responses.
+		/// </summary>
+		ResponseCodes          = 1 << 4,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2449">SASL</a> authentication
+		/// extension, allowing the client to authenticate using the advertized authentication mechanisms
+		/// (see <see cref="Pop3Client.AuthenticationMechanisms"/>).
+		/// </summary>
+		Sasl                   = 1 << 5,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc2595">STLS</a> extension,
+		/// allowing clients to switch to an encrypted SSL/TLS connection after connecting.
+		/// </summary>
+		StartTLS               = 1 << 6,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc1939#page-11">TOP</a> command,
+		/// allowing clients to fetch the headers plus an arbitrary number of lines.
+		/// </summary>
+		Top                    = 1 << 7,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc1939#page-12">UIDL</a> command,
+		/// allowing the client to refer to messages via a UID as opposed to a sequence ID.
+		/// </summary>
+		UIDL                   = 1 << 8,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc1939#page-13">USER</a>
+		/// authentication command, allowing the client to authenticate via a plain-text username
+		/// and password command (not recommended unless no other authentication mechanisms exist).
+		/// </summary>
+		User                   = 1 << 9,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6856">UTF8</a> extension,
+		/// allowing clients to retrieve messages in the UTF-8 encoding.
+		/// </summary>
+		UTF8                   = 1 << 10,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6856">UTF8=USER</a> extension,
+		/// allowing clients to authenticate using UTF-8 encoded usernames and passwords.
+		/// </summary>
+		UTF8User               = 1 << 11,
+
+		/// <summary>
+		/// The server supports the <a href="https://tools.ietf.org/html/rfc6856">LANG</a> extension,
+		/// allowing clients to specify which language the server should use for error strings.
+		/// </summary>
+		Lang                   = 1 << 12,
+	}
+}

src/MailKit/Net/Pop3/Pop3Command.cs

@@ -0,0 +1,74 @@
+//
+// Pop3Command.cs
+//
+// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
+//
+// Copyright (c) 2013-2020 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System;
+using System.Text;
+using System.Threading;
+using System.Globalization;
+using System.Threading.Tasks;
+
+namespace MailKit.Net.Pop3 {
+	/// <summary>
+	/// POP3 command handler.
+	/// </summary>
+	/// <remarks>
+	/// All exceptions thrown by the handler are considered fatal and will
+	/// force-disconnect the connection. If a non-fatal error occurs, set
+	/// it on the <see cref="Pop3Command.Exception"/> property.
+	/// </remarks>
+	delegate Task Pop3CommandHandler (Pop3Engine engine, Pop3Command pc, string text, bool doAsync);
+
+	enum Pop3CommandStatus {
+		Queued         = -5,
+		Active         = -4,
+		Continue       = -3,
+		ProtocolError  = -2,
+		Error          = -1,
+		Ok             =  0
+	}
+
+	class Pop3Command
+	{
+		public CancellationToken CancellationToken { get; private set; }
+		public Pop3CommandHandler Handler { get; private set; }
+		public Encoding Encoding { get; private set; }
+		public string Command { get; private set; }
+		public int Id { get; internal set; }
+
+		// output
+		public Pop3CommandStatus Status { get; internal set; }
+		public ProtocolException Exception { get; set; }
+		public string StatusText { get; set; }
+
+		public Pop3Command (CancellationToken cancellationToken, Pop3CommandHandler handler, Encoding encoding, string format, params object[] args)
+		{
+			Command = string.Format (CultureInfo.InvariantCulture, format, args);
+			CancellationToken = cancellationToken;
+			Encoding = encoding;
+			Handler = handler;
+		}
+	}
+}