MimePart Class

public class MimePart : MimeEntity, IMimePart, 
	IMimeEntity, IDisposable

foreach (var attachment in message.Attachments) {
    if (attachment is MessagePart) {
        var fileName = attachment.ContentDisposition?.FileName;
        var rfc822 = (MessagePart) attachment;

        if (string.IsNullOrEmpty (fileName))
            fileName = "attached-message.eml";

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

        using (var stream = File.Create (fileName))
            part.Content.DecodeTo (stream);
    }
}