Convert MailMessage to Raw text

Here's the same solution, but as an extension method to MailMessage.

Some of the reflection overhead is minimized by grabbing the ConstructorInfo and MethodInfo members once in the static context.

/// <summary>
/// Uses reflection to get the raw content out of a MailMessage.
/// </summary>
public static class MailMessageExtensions
{
    private static readonly BindingFlags Flags = BindingFlags.Instance | BindingFlags.NonPublic;
    private static readonly Type MailWriter = typeof(SmtpClient).Assembly.GetType("System.Net.Mail.MailWriter");
    private static readonly ConstructorInfo MailWriterConstructor = MailWriter.GetConstructor(Flags, null, new[] { typeof(Stream) }, null);
    private static readonly MethodInfo CloseMethod = MailWriter.GetMethod("Close", Flags);
    private static readonly MethodInfo SendMethod = typeof(MailMessage).GetMethod("Send", Flags);

    /// <summary>
    /// A little hack to determine the number of parameters that we
    /// need to pass to the SaveMethod.
    /// </summary>
    private static readonly bool IsRunningInDotNetFourPointFive = SendMethod.GetParameters().Length == 3;

    /// <summary>
    /// The raw contents of this MailMessage as a MemoryStream.
    /// </summary>
    /// <param name="self">The caller.</param>
    /// <returns>A MemoryStream with the raw contents of this MailMessage.</returns>
    public static MemoryStream RawMessage(this MailMessage self)
    {
        var result = new MemoryStream();
        var mailWriter = MailWriterConstructor.Invoke(new object[] { result });
        SendMethod.Invoke(self, Flags, null, IsRunningInDotNetFourPointFive ? new[] { mailWriter, true, true } : new[] { mailWriter, true }, null);
        result = new MemoryStream(result.ToArray());
        CloseMethod.Invoke(mailWriter, Flags, null, new object[] { }, null);
        return result;
    }
}

To grab the underlying MemoryStream:

var email = new MailMessage();
using (var m = email.RawMessage()) {
    // do something with the raw message
}

I've implemented logic in MimeKit to allow you to cast a System.Net.Mail.MailMessage into a MimeKit.MimeMessage. Once you do that, you can simply write the message to a stream:

var message = (MimeMessage) CreateSystemNetMailMessage ();
using (var stream = File.Create ("C:\\message.eml"))
    message.WriteTo (stream);

This does not require reflecting into internal methods which means that it isn't dependent on the runtime, making it far more portable that the other answers given so far.