We do support markdown in email, as outlined here: https://beta.groups.io/g/main/message/1318
You'll be forgiven for not knowing about this, since it's so obscure and supported by (I believe) exactly one email client. :-)
Note that we don't do any conversions of those messages before we send them out (so all email recipients see the original markdown when viewing the message in their clients), but we do display them correctly on the website.
Conversely, if you have your Editor Preference set to Markdown and you compose a message on the website, we send that out with both HTML and plain text parts.
Like I said in the referenced message, I'm happy to add support for flagging a message as markdown if we can agree on the magic word/set of characters. If we do, I think it probably makes sense to convert those messages to HTML before sending them out to the group (ie change the current behavior). This would definitely be a niche thing, (for amongst other reasons) because I'm not sure it's even possible to force Gmail and the other web-based clients to send out just a plain text email anymore.