Not sure about the “features” at the end. Specifically Markdown support. This is a client-side issue. If MUAs parsed Markdown, then there is no reason that mark

Not sure about the “features” at the end. Specifically Markdown support. This is a client-side issue. If MUAs parsed Markdown, then there is no reason that markdown couldn’t replace html in the relevant part of a MIME multipart message. The SMTP server only handles ASCII characters; it really doesn’t care how that is presented. It also afford the possibly of plaintext as a fall back. Arguably, you could have all three.