Emacs ofc :) seriously it should not be too much work although org-mode syntax would be even easier, there is a markdown mode here: https://jblevins.org/projects/markdown-mode/ The email part is not something i have done myself but it has been a feature for a very long time and you can find plenty of guides online.

It would first require a standard for Markdown. After that there would be very little stopping anyone from implementing it. I guess a MIME type for standard Markdown would also be nice.

Pretty sure I've said it before, but it would be a nice middle ground between text and all the complexity HTML+CSS brings in (if you want to compete with other HTML clients).