GNOME Evolution supports Markdown natively (I believe either raw or rendered to HTML on send; I use the latter.)
Mailmate on macos solves this very nicely. As a bonus my html mails have never looked better and i get the bonus of writing in text using markdown . Currently evaluating it.
There's no The Markdown that's standardised (or safe enough), that it could be implemented like that unfortunately.
Thank you. I hate when people say “Markdown” as if it was a single language and not a collective term for 100+ kinda similar languages.