Answered: How to create mail message draft

You can make the message read-write by setting the X-Unsent flag into the message headers:

var msg = new MailMessage();
msg.Subject = "Subject";
msg.Headers.Add("X-Unsent", "1");  // <-- that's it
msg.BodyHtml = "Mail body...";

It should work both for .EML and .MSG message formats.