Most of the Content-Something headers cannot be added like this. Please try using the appropriate methods instead, like this:
byte[] content = Encoding.ASCII.GetBytes("<--MESSAGECONTENT-->");
var contentStream = new MemoryStream(content, false);
var mess = new MimeMessage();
mess.ContentDisposition = new ContentDisposition("inline");
mess.SetContent(contentStream, null, "message/disposition-notification", TransferEncoding.SevenBit);
mess.Headers.Add("Original-Message-ID", "<--MESSAGEID-->");
mess.Headers.Add("Disposition", "automatic-action/MDN-sent-automatically;processed");
mess.Headers.Add("Final-Recipient", "rfc822;some@domain.com");
mess.Save(@"c:\temp\mdn-sample.eml");
If you have a sample of the kind of a message you would like to create, please post it as well - we will let you know how to create an equivalent message using Rebex Secure Mail.