Multipart emails have been doing this for as long as I can remember - emails are delivered with an HTML part and a plain text on for compatibility with clients that don't support it. There's no reason you couldn't release your new mail client with text/mailml support, but you'll have to get people to use it.
Also, please use text/markdown instead of inventing yet another weird markup. Bonus points if it's CommonMark (which is much more strictly defined and thus guaranteed to look the same across renderers).
I've wondered about the viability of text/markdown for email from time to time. It would make a very nice sensible default: the sender can write an email with a hierarchy of headers, quoting (exactly the same way done now with text/plain and supported by most clients), emphasis, hyperlinks, lists, etc., all without depriving the receiver of their preferred formatting and font settings. Also, not nearly as verbose as HTML (and not nearly as much crap).
