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MimeKit

Frequently Asked Questions

Are MimeKit and MailKit completely free? Can I use them in my proprietary product(s)?

Yes. MimeKit and MailKit are both completely free and open source. They are both covered under the MIT license.

Why do I get 'The remote certificate is invalid according to the validation procedure' when I try to Connect?

When you get an exception with that error message, it means that the IMAP, POP3 or SMTP server that you are connecting to is using an SSL certificate that is either expired or untrusted by your system.

Often times, mail servers will use self-signed certificates instead of using a certificate that has been signed by a trusted Certificate Authority. When your system is unable to validate the mail server's certificate because it is not signed by a known and trusted Certificate Authority, the above error will occur.

You can work around this problem by supplying a custom RemoteServerCertificateValidationCallback and setting it on the client's ServerCertificateValidationCallback property.

In the most simplest example, you could do something like this (although I would strongly recommend against it in production use):

C#
using (var client = new SmtpClient ()) {
    client.ServerCertificateValidationCallback = (s, c, h, e) => true;

    client.Connect ("smtp.gmail.com", 465, SecureSocketOptions.SslOnConnect);

    client.Authenticate ("username", "password");

    client.Send (message);

    client.Disconnect (true);
}

Most likely you'll want to instead compare the certificate's Thumbprint property to a known value that you have verified at a prior date.

How do I debug SMTP, POP3 and/or IMAP errors?

All of MailKit's client implementations have a constructor that takes a nifty IProtocolLogger interface for logging client/server communications. Out of the box, you can use the handy ProtocolLogger class. Here are some examples of how to use it:

C#
// log to a file called 'imap.log'
var client = new ImapClient (new ProtocolLogger ("imap.log"));

// log to standard output (i.e. the console)
var client = new ImapClient (new ProtocolLogger (Console.OpenStandardOutput ()));

Note: When submitting a protocol log as part of a bug report, make sure to scrub any sensitive information including your authentication credentials. This information will generally be the base64 encoded blob immediately following an AUTHENTICATE or AUTH command (depending on the type of server) or the cleartext username and password strings in a LOGIN command. The only exception to this case is if you are authenticating with NTLM in which case I may need this information, but only if the bug/error is in the authentication step.

Why don't I see some of my messages when accessing GMail with POP3?

By default, GMail's POP3 server does not behave like a standard POP3 server and hides messages from clients (as well as having other non-standard behavior) that have already been viewed.

If you want to configure your GMail POP3 settings to behave the way POP3 is intended to behave, you'll need to log in to your GMail account via your web browser and navigate to the Forwarding and POP/IMAP tab of your GMail Settings page and make the following changes in the POP3 Download section:

  • Enable POP for all mail (even if it has already been downloaded).

  • When messages are accessed with POP, keep GMail's copy in the Inbox.

How do I access GMail using MailKit?

The first thing that you will need to do is to configure your GMail account to enable less secure apps, or you'll need to use OAuth 2.0 authentication (which is a bit more complex).

Then, assuming that your GMail account is user@gmail.com, you would use the following code snippet to connect to GMail via IMAP:

C#
using (var client = new ImapClient ()) {
    client.Connect ("imap.gmail.com", 993, SecureSocketOptions.SslOnConnect);
    client.Authenticate ("user@gmail.com", "password");

    // do stuff...

    client.Disconnect (true);
}

Connecting via POP3 or SMTP is identical except for the host names and ports (and, of course, you'd use a Pop3Client or SmtpClient as appropriate).

If you decide to authenticate via OAuth 2.0 instead of enabling less secure apps, you'll need to read Google's OAuth 2.0 documentation to learn how to do this.

Once you obtain an OAuth2 access token, you can use it with MailKit by using the access token to create a new OAuth2 SASL mechanism context and then authenticating with it.

C#
var certificate = new X509Certificate2 (@"C:\path\to\certificate.p12", "password", X509KeyStorageFlags.Exportable);
var credential = new ServiceAccountCredential (new ServiceAccountCredential.Initializer ("your-developer-id@developer.gserviceaccount.com") {
    // Note: other scopes can be found here: https://developers.google.com/gmail/api/auth/scopes
    Scopes = new[] { "https://mail.google.com/" },
    User = "user@gmail.com"
}.FromCertificate (certificate));

// Note: result will be true if the access token was received successfully
bool result = await credential.RequestAccessTokenAsync (cancel.Token);

// create an OAuth2 SASL context
var oauth2 = new SaslMechanismOAuth2 ("user@gmail.com", credential.Token.AccessToken);

// authenticate using the OAuth2 SASL mechanism
client.Authenticate (oauth2);
How do I create a message with attachments?

To construct a message with attachments, the first thing you'll need to do is create a multipart/mixed container which you'll then want to add the message body to first. Once you've added the body, you can then add MIME parts to it that contain the content of the files you'd like to attach, being sure to set the Content-Disposition header value to attachment. You'll probably also want to set the filename parameter on the Content-Disposition header as well as the name parameter on the Content-Type header. The most convenient way to do this is to use the MimePartFileName property which will set both parameters for you as well as setting the Content-Disposition header value to attachment if it has not already been set to something else.

C#
var message = new MimeMessage ();
message.From.Add (new MailboxAddress ("Joey", "joey@friends.com"));
message.To.Add (new MailboxAddress ("Alice", "alice@wonderland.com"));
message.Subject = "How you doin?";

// create our message text, just like before (except don't set it as the message.Body)
var body = new TextPart ("plain") {
    Text = @"Hey Alice,

What are you up to this weekend? Monica is throwing one of her parties on
Saturday and I was hoping you could make it.

Will you be my +1?

-- Joey
"
};

// create an image attachment for the file located at path
var attachment = new MimePart ("image", "gif") {
    Content = new MimeContent (File.OpenRead (path), ContentEncoding.Default),
    ContentDisposition = new ContentDisposition (ContentDisposition.Attachment),
    ContentTransferEncoding = ContentEncoding.Base64,
    FileName = Path.GetFileName (path)
};

// now create the multipart/mixed container to hold the message text and the
// image attachment
var multipart = new Multipart ("mixed");
multipart.Add (body);
multipart.Add (attachment);

// now set the multipart/mixed as the message body
message.Body = multipart;

A simpler way to construct messages with attachments is to take advantage of the BodyBuilder class.

C#
            var message = new MimeMessage ();
            message.From.Add (new MailboxAddress ("Joey", "joey@friends.com"));
            message.To.Add (new MailboxAddress ("Alice", "alice@wonderland.com"));
            message.Subject = "How you doin?";

            var builder = new BodyBuilder ();

            // Set the plain-text version of the message text
            builder.TextBody = @"Hey Alice,

What are you up to this weekend? Monica is throwing one of her parties on
Saturday and I was hoping you could make it.

Will you be my +1?

-- Joey
";

            // We may also want to attach a calendar event for Monica's party...
            builder.Attachments.Add (@"C:\Users\Joey\Documents\party.ics");

            // Now we just need to set the message body and we're done
            message.Body = builder.ToMessageBody ();

For more information, see Creating Messages.

How do I get the main body of a message?

(Note: for the TL;DR version, skip to the end)

MIME is a tree structure of parts. There are multiparts which contain other parts (even other multiparts). There are message parts which contain messages. And finally, there are leaf-node parts which contain content.

There are a few common message structures:

  1. The message contains only a text/plain or text/html part (easy, just use that).

  2. The message contains a multipart/alternative which will typically look a bit like this:

    multipart/alternative
      text/plain
      text/html

  3. Same as above, but the html part is inside a multipart/related so that it can embed images:

    multipart/alternative
      text/plain
      multipart/related
        text/html
        image/jpeg
        image/png

  4. The message contains a textual body part as well as some attachments:

    multipart/mixed
      text/plain
      application/octet-stream
      application/zip

  5. The same as above, but with the first part replaced with either #2 or #3 To illustrate:

    multipart/mixed
      multipart/alternative
        text/plain
        text/html
      application/octet-stream
      application/zip

    Or:

    multipart/mixed
      multipart/alternative
        text/plain
        multipart/related
          text/html
          image/jpeg
          image/png
      application/octet-stream
      application/zip

For your convenience, the MimeMessage class has 2 properties that you may find useful: TextBody and HtmlBody.

For more information, see [workingwithmessages].

How do I tell if a message has attachments?

In most cases, a message with a body that has a MIME-type of multipart/mixed containing more than a single part probably has attachments. As illustrated above, the first part of a multipart/mixed is typically the textual body of the message, but it is not always quite that simple.

In general, MIME attachments will have a Content-Disposition header with a value of attachment. To get the list of body parts matching this criteria, you can use the MimeMessageAttachments property.

Unfortunately, not all mail clients follow this convention and so you may need to write your own custom logic. For example, you may wish to treat all body parts having a name or filename parameter set on them:

C#
var attachments = message.BodyParts.OfType<MimePart> ().Where (part => !string.IsNullOrEmpty (part.FileName));

A more sophisticated approach is to treat body parts not referenced by the main textual body part of the message as attachments. In other words, treat any body part not used for rendering the message as an attachment. For an example on how to do this, consider the following code snippets:

C#
/// <summary>
/// Visits a MimeMessage and generates HTML suitable to be rendered by a browser control.
/// </summary>
class HtmlPreviewVisitor : MimeVisitor
{
    List<MultipartRelated> stack = new List<MultipartRelated> ();
    List<MimeEntity> attachments = new List<MimeEntity> ();
    readonly string tempDir;
    string body;

    /// <summary>
    /// Creates a new HtmlPreviewVisitor.
    /// </summary>
    /// <param name="tempDirectory">A temporary directory used for storing image files.</param>
    public HtmlPreviewVisitor (string tempDirectory)
    {
        tempDir = tempDirectory;
    }

    /// <summary>
    /// The list of attachments that were in the MimeMessage.
    /// </summary>
    public IList<MimeEntity> Attachments {
        get { return attachments; }
    }

    /// <summary>
    /// The HTML string that can be set on the BrowserControl.
    /// </summary>
    public string HtmlBody {
        get { return body ?? string.Empty; }
    }

    protected override void VisitMultipartAlternative (MultipartAlternative alternative)
    {
        // walk the multipart/alternative children backwards from greatest level of faithfulness to the least faithful
        for (int i = alternative.Count - 1; i >= 0 && body == null; i--)
            alternative[i].Accept (this);
    }

    protected override void VisitMultipartRelated (MultipartRelated related)
    {
        var root = related.Root;

        // push this multipart/related onto our stack
        stack.Add (related);

        // visit the root document
        root.Accept (this);

        // pop this multipart/related off our stack
        stack.RemoveAt (stack.Count - 1);
    }

    // look up the image based on the img src url within our multipart/related stack
    bool TryGetImage (string url, out MimePart image)
    {
        UriKind kind;
        int index;
        Uri uri;

        if (Uri.IsWellFormedUriString (url, UriKind.Absolute))
            kind = UriKind.Absolute;
        else if (Uri.IsWellFormedUriString (url, UriKind.Relative))
            kind = UriKind.Relative;
        else
            kind = UriKind.RelativeOrAbsolute;

        try {
            uri = new Uri (url, kind);
        } catch {
            image = null;
            return false;
        }

        for (int i = stack.Count - 1; i >= 0; i--) {
            if ((index = stack[i].IndexOf (uri)) == -1)
                continue;

            image = stack[i][index] as MimePart;
            return image != null;
        }

        image = null;

        return false;
    }

    // Save the image to our temp directory and return a "file://" url suitable for
    // the browser control to load.
    // Note: if you'd rather embed the image data into the HTML, you can construct a
    // "data:" url instead.
    string SaveImage (MimePart image, string url)
    {
        string fileName = url.Replace (':', '_').Replace ('\\', '_').Replace ('/', '_');

        string path = Path.Combine (tempDir, fileName);

        if (!File.Exists (path)) {
            using (var output = File.Create (path))
                image.Content.DecodeTo (output);
        }

        return "file://" + path.Replace ('\\', '/');
    }

    // Replaces <img src=...> urls that refer to images embedded within the message with
    // "file://" urls that the browser control will actually be able to load.
    void HtmlTagCallback (HtmlTagContext ctx, HtmlWriter htmlWriter)
    {
        if (ctx.TagId == HtmlTagId.Image && !ctx.IsEndTag && stack.Count > 0) {
            ctx.WriteTag (htmlWriter, false);

            // replace the src attribute with a file:// URL
            foreach (var attribute in ctx.Attributes) {
                if (attribute.Id == HtmlAttributeId.Src) {
                    MimePart image;
                    string url;

                    if (!TryGetImage (attribute.Value, out image)) {
                        htmlWriter.WriteAttribute (attribute);
                        continue;
                    }

                    url = SaveImage (image, attribute.Value);

                    htmlWriter.WriteAttributeName (attribute.Name);
                    htmlWriter.WriteAttributeValue (url);
                } else {
                    htmlWriter.WriteAttribute (attribute);
                }
            }
        } else if (ctx.TagId == HtmlTagId.Body && !ctx.IsEndTag) {
            ctx.WriteTag (htmlWriter, false);

            // add and/or replace oncontextmenu="return false;"
            foreach (var attribute in ctx.Attributes) {
                if (attribute.Name.ToLowerInvariant () == "oncontextmenu")
                    continue;

                htmlWriter.WriteAttribute (attribute);
            }

            htmlWriter.WriteAttribute ("oncontextmenu", "return false;");
        } else {
            // pass the tag through to the output
            ctx.WriteTag (htmlWriter, true);
        }
    }

    protected override void VisitTextPart (TextPart entity)
    {
        TextConverter converter;

        if (body != null) {
            // since we've already found the body, treat this as an attachment
            attachments.Add (entity);
            return;
        }

        if (entity.IsHtml) {
            converter = new HtmlToHtml {
                HtmlTagCallback = HtmlTagCallback
            };
        } else if (entity.IsFlowed) {
            var flowed = new FlowedToHtml ();
            string delsp;

            if (entity.ContentType.Parameters.TryGetValue ("delsp", out delsp))
                flowed.DeleteSpace = delsp.ToLowerInvariant () == "yes";

            converter = flowed;
        } else {
            converter = new TextToHtml ();
        }

        body = converter.Convert (entity.Text);
    }

    protected override void VisitTnefPart (TnefPart entity)
    {
        // extract any attachments in the MS-TNEF part
        attachments.AddRange (entity.ExtractAttachments ());
    }

    protected override void VisitMessagePart (MessagePart entity)
    {
        // treat message/rfc822 parts as attachments
        attachments.Add (entity);
    }

    protected override void VisitMimePart (MimePart entity)
    {
        // realistically, if we've gotten this far, then we can treat this as an attachment
        // even if the IsAttachment property is false.
        attachments.Add (entity);
    }
}

And the way you'd use this visitor might look something like this:

C#
void Render (MimeMessage message)
{
    var tmpDir = Path.Combine (Path.GetTempPath (), message.MessageId);
    var visitor = new HtmlPreviewVisitor (tmpDir);

    Directory.CreateDirectory (tmpDir);

    message.Accept (visitor);

    DisplayHtml (visitor.HtmlBody);
    DisplayAttachments (visitor.Attachments);
}

Once you've rendered the message using the above technique, you'll have a list of attachments that were not used, even if they did not match the simplistic criteria used by the MimeMessageAttachments property.

Why doesn't the MimeMessage class implement ISerializable so that I can serialize a message to disk and read it back later?

The MimeKit API was designed to use the existing MIME format for serialization. In light of this, the ability to use the .NET serialization API and format did not make much sense to support.

You can easily serialize a MimeMessage to a stream using the WriteTo methods.

How do I save messages?

One you've got a MimeMessage, you can save it to a file using the WriteTo method:

C#
message.WriteTo ("message.eml");

The WriteTo method also has overloads that allow you to write the message to a Stream instead.

By default, the WriteTo method will save the message using DOS line-endings on Windows and Unix line-endings on Unix-based systems such as macOS and Linux. You can override this behavior by passing a FormatOptions argument to the method:

C#
// clone the default formatting options
var format = FormatOptions.Default.Clone ();

// override the line-endings to be DOS no matter what platform we are on
format.NewLineFormat = NewLineFormat.Dos;

message.WriteTo (format, "message.eml");
How do I save attachments?

If you've already got a MimePart that represents the attachment that you'd like to save, here's how you might save it:

C#
using (var stream = File.Create (fileName))
    attachment.Content.DecodeTo (stream);

Pretty simple, right?

But what if your attachment is actually a MessagePart?

To save the content of a message/rfc822 part, you'd use the following code snippet:

C#
using (var stream = File.Create (fileName))
    attachment.Message.WriteTo (stream);

If you are iterating over all of the attachments in a message, you might do something like this:

C#
foreach (var attachment in message.Attachments) {
    if (attachment is MessagePart) {
        var fileName = attachment.ContentDisposition?.FileName :
            (attachment.ContentType.Name ?? "attached.eml");
        var rfc822 = (MessagePart) attachment;

        rfc822.Message.WriteTo (stream);
    } else {
        var part = (MimePart) attachment;
        var fileName = part.FileName;

        using (var stream = File.Create (fileName))
            part.Content.DecodeTo (stream);
    }
}
How do I get the email addresses in the From, To, and Cc headers?

The From, To, and Cc properties of a MimeMessage are all of type InternetAddressList. An InternetAddressList is a list of InternetAddress items. This is where some people start to get lost, conceptually, because InternetAddress is an abstract class that only really has a Name property.

As you've probably already discovered, the Name property contains the name of the person (if available), but what you probably want is his or her email address, not their name. So how do you get it?

To get the email address, you'll need to figure out what subclass of address each InternetAddress really is. There are 2 direct subclasses of InternetAddress. They are: GroupAddress and MailboxAddress.

A GroupAddress is a named group of more InternetAddress items that are contained within the Members property. To get an idea of what a group address represents, consider the following examples:

To: My Friends: Joey <joey@friends.com>, Monica <monica@friends.com>, "Mrs. Chanandler Bong" <chandler@friends.com>, Ross <ross@friends.com>, Rachel <rachel@friends.com>;

In the above example, the To header's InternetAddressList will contain only 1 item which will be a GroupAddress with a Name value of My Friends. The Members property of the GroupAddress will contain 5 more InternetAddress items (which will all be instances of MailboxAddress).

The above example, however, is not very likely to ever be seen in messages you deal with. A far more common example would be the one below:

To: undisclosed-recipients:;

Most of the time, the From, To, and Cc headers will only contain mailbox addresses. As you will notice, a MailboxAddress has an Address property which will contain the email address of the mailbox. In the following example, the Address property will contain the value john@smith.com:

To: John Smith <john@smith.com>

If you only care about getting a flattened list of the mailbox addresses in one of the address headers, you can do something like this:

C#
foreach (var mailbox in message.To.Mailboxes)
    Console.WriteLine ("{0}'s email address is {1}", mailbox.Name, mailbox.Address);
Why do attachments with unicode filenames appear as "ATT0####.dat" in Outlook?

An attachment filename is stored as a MIME parameter on the Content-Disposition header. Unfortunately, the original MIME specifications did not specify a method for encoding non-ascii filenames. In 1997, rfc2184 (later updated by rfc2231) was published which specified an encoding mechanism to use for encoding them. Since there was a window in time where the MIME specifications did not define a way to encode them, some mail client developers decided to use the mechanism described by rfc2047 which was meant for encoding non-ASCII text in headers. While this may at first seem logical, the problem with this approach was that rfc2047 encoded-word tokens are not allowed to be in quotes (as well as some other issues) and so another, more appropriate, encoding mechanism was needed.

Outlook is one of those mail clients which decided to encode filenames using the mechanism described in rfc2047 and until Outlook 2007, did not support filenames encoded using the mechanism defined in rfc2231.

As of MimeKit v1.2.18, it is possible to configure MimeKit to use the rfc2047 encoding mechanism for filenames in the following two ways:

The first way is to set the encoding method on each individual Parameter:

C#
Parameter param;

if (attachment.ContentDisposition.Parameters.TryGetValue ("filename", out param))
    param.EncodingMethod = ParameterEncodingMethod.Rfc2047;

The other way is to use a FormatOptions:

C#
var options = FormatOptions.Default.Clone ();
options.ParameterEncodingMethod = ParameterEncodingMethod.Rfc2047;

message.WriteTo (options, stream);
How do I decrypt PGP messages that are embedded in the main message text?

Some PGP-enabled mail clients, such as Thunderbird, embed encrypted PGP blurbs within the text/plain body of the message rather than using the PGP/MIME format that MimeKit prefers.

These messages often look something like this:

Return-Path: <pgp-enthusiast@example.com>
Received: from [127.0.0.1] (hostname.example.com. [201.95.8.17])
    by mx.google.com with ESMTPSA id l67sm26628445yha.8.2014.04.27.13.49.44
    for <pgp-enthusiast@example.com>
    (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128);
    Sun, 27 Apr 2014 13:49:44 -0700 (PDT)
Message-ID: <535D6D67.8020803@example.com>
Date: Sun, 27 Apr 2014 17:49:43 -0300
From: Die-Hard PGP Fan <pgp-enthusiast@example.com>
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0
MIME-Version: 1.0
To: undisclosed-recipients:;
Subject: Test of inline encrypted PGP blocks
X-Enigmail-Version: 1.6
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
X-Antivirus: avast! (VPS 140427-1, 27/04/2014), Outbound message
X-Antivirus-Status: Clean

-----BEGIN PGP MESSAGE-----
Charset: ISO-8859-1
Version: GnuPG v2.0.22 (MingW32)
Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/

SGFoISBJIGZvb2xlZCB5b3UsIHRoaXMgdGV4dCBpc24ndCBhY3R1YWxseSBlbmNy
eXB0ZWQgd2l0aCBQR1AsCml0J3MgYWN0dWFsbHkgb25seSBiYXNlNjQgZW5jb2Rl
ZCEKCkknbSBqdXN0IHVzaW5nIHRoaXMgYXMgYW4gZXhhbXBsZSwgdGhvdWdoLCBz
byBpdCBkb2Vzbid0IHJlYWxseSBtYXR0ZXIuCgpGb3IgdGhlIHNha2Ugb2YgYXJn
dW1lbnQsIHdlJ2xsIHByZXRlbmQgdGhhdCB0aGlzIGlzIGFjdHVhbGx5IGFuIGVu
Y3J5cHRlZApibHVyYi4gTW1ta2F5PyBUaGFua3MuCg==
-----END PGP MESSAGE-----

To deal with these kinds of messages, MimeKit's OpenPgpContext includes a GetDecryptedStream method which can be used to get the raw decrypted stream.

The method variant that has a DigitalSignatureCollection output parameter is useful in cases where the encrypted PGP blurb is also digitally signed, allowing you to get your hands on the list of digitial signatures in order for you to verify each of them.

To decrypt the sample message above, you could use the following code snippet:

C#
static Stream Decrypt (MimeMessage message)
{
    var text = message.TextBody;

    using (var memory = new MemoryStream (Encoding.ASCII.GetBytes (text), false)) {
        using (var ctx = new MyGnuPGContext ()) {
            return ctx.GetDecryptedStream (memory);
        }
    }
}
How do I reply to a message using MimeKit?

Replying to a message is fairly simple. For the most part, you'd just create the reply message the same way you'd create any other message. There are only a few slight differences:

  1. In the reply message, you'll want to prefix the Subject header with "Re: " if the prefix doesn't already exist in the message you are replying to (in other words, if you are replying to a message with a Subject of "Re: party tomorrow night!", you would not prefix it with another "Re: ").

  2. You will want to set the reply message's In-Reply-To header to the value of the Message-Id header in the original message.

  3. You will want to copy the original message's References header into the reply message's References header and then append the original message's Message-Id header.

  4. You will probably want to "quote" the original message's text in the reply.

If this logic were to be expressed in code, it might look something like this:

C#
public static MimeMessage Reply (MimeMessage message, MailboxAddress from, bool replyToAll)
{
    var reply = new MimeMessage ();

    reply.From.Add (from);

    // reply to the sender of the message
    if (message.ReplyTo.Count > 0) {
        reply.To.AddRange (message.ReplyTo);
    } else if (message.From.Count > 0) {
        reply.To.AddRange (message.From);
    } else if (message.Sender != null) {
        reply.To.Add (message.Sender);
    }

    if (replyToAll) {
        // include all of the other original recipients (removing ourselves from the list)
        reply.To.AddRange (message.To.Mailboxes.Where (x => x.Address != from.Address));
        reply.Cc.AddRange (message.Cc.Mailboxes.Where (x => x.Address != from.Address));
    }

    // set the reply subject
    if (!message.Subject.StartsWith ("Re:", StringComparison.OrdinalIgnoreCase))
        reply.Subject = "Re: " + message.Subject;
    else
        reply.Subject = message.Subject;

    // construct the In-Reply-To and References headers
    if (!string.IsNullOrEmpty (message.MessageId)) {
        reply.InReplyTo = message.MessageId;
        foreach (var id in message.References)
            reply.References.Add (id);
        reply.References.Add (message.MessageId);
    }

    // quote the original message text
    using (var quoted = new StringWriter ()) {
        var sender = message.Sender ?? message.From.Mailboxes.FirstOrDefault ();
        var name = sender != null ? (!string.IsNullOrEmpty (sender.Name) ? sender.Name : sender.Address) : "someone";

        quoted.WriteLine ("On {0}, {1} wrote:", message.Date.ToString ("f"), name);
        using (var reader = new StringReader (message.TextBody)) {
            string line;

            while ((line = reader.ReadLine ()) != null) {
                quoted.Write ("> ");
                quoted.WriteLine (line);
            }
        }

        reply.Body = new TextPart ("plain") {
            Text = quoted.ToString ()
        };
    }

    return reply;
}

But what if you wanted to reply to a message and quote the HTML formatting of the original message body (assuming it has an HTML body) while still including the embedded images?

This gets a bit more complicated, but it's still doable...

The first thing we'd need to do is implement our own MimeVisitor to handle this:

C#
public class ReplyVisitor : MimeVisitor
{
    readonly Stack<Multipart> stack = new Stack<Multipart> ();
    MimeMessage message;
    MimeEntity body;

    /// <summary>
    /// Creates a new ReplyVisitor.
    /// </summary>
    public ReplyVisitor ()
    {
    }

    /// <summary>
    /// Gets the reply.
    /// </summary>
    /// <value>The reply.</value>
    public MimeEntity Body {
        get { return body; }
    }

    void Push (MimeEntity entity)
    {
        var multipart = entity as Multipart;

        if (body == null) {
            body = entity;
        } else {
            var parent = stack.Peek ();
            parent.Add (entity);
        }

        if (multipart != null)
            stack.Push (multipart);
    }

    void Pop ()
    {
        stack.Pop ();
    }

    public static string GetOnDateSenderWrote (MimeMessage message)
    {
        var sender = message.Sender != null ? message.Sender : message.From.Mailboxes.FirstOrDefault ();
        var name = sender != null ? (!string.IsNullOrEmpty (sender.Name) ? sender.Name : sender.Address) : "someone";

        return string.Format ("On {0}, {1} wrote:", message.Date.ToString ("f"), name);
    }

    /// <summary>
    /// Visit the specified message.
    /// </summary>
    /// <param name="message">The message.</param>
    public override void Visit (MimeMessage message)
    {
        this.message = message;
        stack.Clear ();

        base.Visit (message);
    }

    protected override void VisitMultipartAlternative (MultipartAlternative alternative)
    {
        var multipart = new MultipartAlternative ();

        Push (multipart);

        for (int i = 0; i < alternative.Count; i++)
            alternative[i].Accept (this);

        Pop ();
    }

    protected override void VisitMultipartRelated (MultipartRelated related)
    {
        var multipart = new MultipartRelated ();
        var root = related.Root;

        Push (multipart);

        root.Accept (this);

        for (int i = 0; i < related.Count; i++) {
            if (related[i] != root)
                related[i].Accept (this);
        }

        Pop ();
    }

    protected override void VisitMultipart (Multipart multipart)
    {
        foreach (var part in multipart) {
            if (part is MultipartAlternative)
                part.Accept (this);
            else if (part is MultipartRelated)
                part.Accept (this);
            else if (part is TextPart)
                part.Accept (this);
        }
    }

    void HtmlTagCallback (HtmlTagContext ctx, HtmlWriter htmlWriter)
    {
        if (ctx.TagId == HtmlTagId.Body && !ctx.IsEmptyElementTag) {
            if (ctx.IsEndTag) {
                // end our opening <blockquote>
                htmlWriter.WriteEndTag (HtmlTagId.BlockQuote);

                // pass the </body> tag through to the output
                ctx.WriteTag (htmlWriter, true);
            } else {
                // pass the <body> tag through to the output
                ctx.WriteTag (htmlWriter, true);

                // prepend the HTML reply with "On {DATE}, {SENDER} wrote:"
                htmlWriter.WriteStartTag (HtmlTagId.P);
                htmlWriter.WriteText (GetOnDateSenderWrote (message));
                htmlWriter.WriteEndTag (HtmlTagId.P);

                // Wrap the original content in a <blockquote>
                htmlWriter.WriteStartTag (HtmlTagId.BlockQuote);
                htmlWriter.WriteAttribute (HtmlAttributeId.Style, "border-left: 1px #ccc solid; margin: 0 0 0 .8ex; padding-left: 1ex;");

                ctx.InvokeCallbackForEndTag = true;
            }
        } else {
            // pass the tag through to the output
            ctx.WriteTag (htmlWriter, true);
        }
    }

    string QuoteText (string text)
    {
        using (var quoted = new StringWriter ()) {
            quoted.WriteLine (GetOnDateSenderWrote (message));

            using (var reader = new StringReader (text)) {
                string line;

                while ((line = reader.ReadLine ()) != null) {
                    quoted.Write ("> ");
                    quoted.WriteLine (line);
                }
            }

            return quoted.ToString ();
        }
    }

    protected override void VisitTextPart (TextPart entity)
    {
        string text;

        if (entity.IsHtml) {
            var converter = new HtmlToHtml {
                HtmlTagCallback = HtmlTagCallback
            };

            text = converter.Convert (entity.Text);
        } else if (entity.IsFlowed) {
            var converter = new FlowedToText ();

            text = converter.Convert (entity.Text);
            text = QuoteText (text);
        } else {
            // quote the original message text
            text = QuoteText (entity.Text);
        }

        var part = new TextPart (entity.ContentType.MediaSubtype.ToLowerInvariant ()) {
            Text = text
        };

        Push (part);
    }

    protected override void VisitMessagePart (MessagePart entity)
    {
        // don't descend into message/rfc822 parts
    }
}
C#
public static MimeMessage Reply (MimeMessage message, MailboxAddress from, bool replyToAll)
{
    var visitor = new ReplyVisitor ();
    var reply = new MimeMessage ();

    reply.From.Add (from);

    // reply to the sender of the message
    if (message.ReplyTo.Count > 0) {
        reply.To.AddRange (message.ReplyTo);
    } else if (message.From.Count > 0) {
        reply.To.AddRange (message.From);
    } else if (message.Sender != null) {
        reply.To.Add (message.Sender);
    }

    if (replyToAll) {
        // include all of the other original recipients (removing ourselves from the list)
        reply.To.AddRange (message.To.Mailboxes.Where (x => x.Address != from.Address));
        reply.Cc.AddRange (message.Cc.Mailboxes.Where (x => x.Address != from.Address));
    }

    // set the reply subject
    if (!message.Subject.StartsWith ("Re:", StringComparison.OrdinalIgnoreCase))
        reply.Subject = "Re: " + message.Subject;
    else
        reply.Subject = message.Subject;

    // construct the In-Reply-To and References headers
    if (!string.IsNullOrEmpty (message.MessageId)) {
        reply.InReplyTo = message.MessageId;
        foreach (var id in message.References)
            reply.References.Add (id);
        reply.References.Add (message.MessageId);
    }

    visitor.Visit (message);

    reply.Body = visitor.Body ?? new TextPart ("plain") { Text = ReplyVisitor.GetOnDateSenderWrote (message) + Environment.NewLine };

    return reply;
}
How do I forward a message?

There are 2 common ways of forwarding a message: attaching the original message as an attachment and inlining the message body much like replying typically does. Which method you choose is up to you.

To forward a message by attaching it as an attachment, you would do do something like this:

C#
public static MimeMessage Forward (MimeMessage original, MailboxAddress from, IEnumerable<InternetAddress> to)
{
    var message = new MimeMessage ();
    message.From.Add (from);
    message.To.AddRange (to);

    // set the forwarded subject
    if (!original.Subject.StartsWith ("FW:", StringComparison.OrdinalIgnoreCase))
        message.Subject = "FW: " + original.Subject;
    else
        message.Subject = original.Subject;

    // create the main textual body of the message
    var text = new TextPart ("plain") { Text = "Here's the forwarded message:" };

    // create the message/rfc822 attachment for the original message
    var rfc822 = new MessagePart { Message = original };

    // create a multipart/mixed container for the text body and the forwarded message
    var multipart = new Multipart ("mixed");
    multipart.Add (text);
    multipart.Add (rfc822);

    // set the multipart as the body of the message
    message.Body = multipart;

    return message;
}

To forward a message by inlining the original message's text content, you can do something like this:

C#
public static MimeMessage Forward (MimeMessage original, MailboxAddress from, IEnumerable<InternetAddress> to)
{
    var message = new MimeMessage ();
    message.From.Add (from);
    message.To.AddRange (to);

    // set the forwarded subject
    if (!original.Subject.StartsWith ("FW:", StringComparison.OrdinalIgnoreCase))
        message.Subject = "FW: " + original.Subject;
    else
        message.Subject = original.Subject;

    // quote the original message text
    using (var text = new StringWriter ()) {
        text.WriteLine ();
        text.WriteLine ("-----Original Message-----");
        test.WriteLine ("From: {0}", original.From);
        text.WriteLine ("Sent: {0}", DateUtils.FormatDate (original.Date));
        text.WriteLine ("To: {0}", original.To);
        text.WriteLine ("Subject: {0}", original.Subject);
        text.WriteLine ();

        text.Write (original.TextBody);

        message.Body = new TextPart ("plain") {
            Text = text.ToString ()
        };
    }

    return message;
}
How do I get all of the unread messages in a folder?

The easiest way is to search for all of the messages that do not have the \Seen flag on them, like so:

C#
foreach (var uid in folder.Search (SearchQuery.NotSeen)) {
    var message = folder.GetMessage (uid);
}
How would I parse multipart/form-data from an HTTP web request?

Since classes like HttpWebRequest take care of parsing the HTTP headers (which includes the Content-Type header) and only offer a content stream to consume, MimeKit provides a way to deal with this using the following two static methods: MimeEntityLoad(ParserOptions, ContentType, Stream, CancellationToken) and MimeEntityLoad(ContentType, Stream, CancellationToken)

Here's how you might use these methods:

C#
MimeEntity ParseMultipartFormData (HttpWebResponse response)
{
    var contentType = ContentType.Parse (response.ContentType);

    return MimeEntity.Parse (contentType, response.GetResponseStream ());
}

If the multipart/form-data HTTP response is expected to be large and you do not wish for the content to be read into memory, you can use the following approach:

C#
MimeEntity ParseMultipartFormData (HttpWebResponse response)
{
    // create a temporary file to store our large HTTP data stream
    var tmp = Path.GetTempFileName ();

    using (var stream = File.Open (tmp, FileMode.Open, FileAccess.ReadWrite)) {
        // create a header for the multipart/form-data MIME entity based on the Content-Type value of the HTTP
        // response
        var header = Encoding.UTF8.GetBytes (string.Format ("Content-Type: {0}\r\n\r\n", response.ContentType));

        // write the header to the stream
        stream.Write (header, 0, header.Length);

        // copy the content of the HTTP response to our temporary stream
        response.GetResponseStream ().CopyTo (stream);

        // reset the stream back to the beginning
        stream.Position = 0;

        // parse the MIME entity with persistent = true, telling the parser not to load the content into memory
        return MimeEntity.Load (stream, persistent: true);
    }
}