You can feed the RTF text into the MailMessage class via Attachments collection like this:
byte[] rtf; // source RTF text as array of bytes
MailMessage mail = new MailMessage();
Attachment att = new Attachment();
att.Options = Rebex.Mime.MimeOptions.AllowAnyTextCharacters;
att.SetContent(new MemoryStream(rtf), "file.rtf", "application/rtf");
mail.Attachments.Add(att);
then you would have to save and reload the mail to raise Rtf to Html conversion like this:
MemoryStream ms = new MemoryStream();
mail.Save(ms);
ms.Position = 0;
mail.Load(ms);
finally you can find the converted HTML in BodyHtml property (mail.BodyHtml), so if you send the email now, it will contain the HTML version of the original RTF document you supplied which will make it possible to read the mail by most email clients.