Actually, the result of both Rebex MailMessage.Save and .ToByteArray is a representation of the message in MIME format, which is well-defined and widely supported. Almost any email client can read it (Outlook 2010, Outlook Express, Windows Mail, Mozilla Thunderbird, etc.) and third-party mail components can read it (use .eml extension).
The only problem is with System MailMessage, which doesn't have Load or Save methods (actually, Save can be simulated by 'sending' it to a specified pickup folder, but you can't load the result back into System MailMessage), so its usefulness is very limited.