Earlier quoted context omitted.
I'm surprised we haven't seen the mIRC's of email yet (or maybe there have but that just didn't work?). mIRC was a popular client for IRC on Windows. It extended IRC with features such as color and formatting in your messages, which was beyond the protocol. A feature like that worked just fine because the color codes were mostly invisible to those who weren't using a client that supported them. In this way, mIRC and…
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).