I'm intrigued by that idea. Markdown or Markdown-like subsets have proven to be sufficient for communication in a variety of media (Slack, Wikipedia, forum comments, GitHub issues/comments, etc.)

Email clients are a bit of an oligopoly, so execution would depend on which player(s) are involved. Most of these initiatives would need an "everyone but Outlook" coalition to succeed.

A few thoughts on implementation:

- Multipart MIME is still a big part of HTML mail, so it would make sense to piggypack on that support. Instead of adding a new MIME attachment for Markdown, it would make sense to extend the existing support for text/plain renditions and find some way to signal that it's Markdown. (e.g. text/plain; charset=UTF-8; variant=MarkdownMail)

- For mail clients that aren't Markdown-aware, it would be important to produce an HTML-rendered variant with a reasonable style sheet.

- A Markdown-first mail client would either need to limit formatting to Markdown or soft-block access to non-Markdown formatting. ("Custom text colors will not be visible on some devices and may be lost when replying. Continue?")

- When viewing a Markdown-enabled message, a Markdown-first mail client would render the text/plain variant in its preferred style sheet, not the HTML-rendered variant.

- When replying to a Markdown-enabled message, a Markdown-first mail client would use the text/plain variant as the source of truth for quoted replies, not the HTML-rendered variant.

A few tricky areas:

- Mail clients would need to agree on a Markdown dialect, especially for extensions like tables.

- Markdown isn't very incompatible with quoting. There would need to be a solution for inline quoting.

- For everyone's sanity, it would seem important to agree on a delimiter to separate prior messages and their headers.

- To not be worse than HTML mail, we'd really need a way to signal that a paragraph is part of a mail signature so that it can be rendered in a subtle way. As much as we might wish that mail signatures would just go away, they will continue to be used, and rendering them as standard paragraph text just makes them more distracting.