General users want WYSIWYG. There's nothing wrong with HTML. It is the predominant standard and along with CSS, very expressive.
Send plain text emails. Use * and _ to emphasize things. This was something people have been doing for damn near 50 years now.
I'd say Thunderbird is a pretty good email client
Back in the day I wrote a modal console-based mail reader, which was much more flexible than the alternatives at the time (mutt, pine, etc).
I wrote the core in C++ and used Lua for the UI and all scripting stuff. The user-interface, processing filtering, hooks, etc, were all based on lua and it was pretty good, and extremely quick to use if you had it setup like you wanted.
But to go back to the topic a lot of the console clients, such as mutt, use "mailcap" files. They allow you to filter the content of messages based on mime-types. So adding the ability to read/send markdown would be trivial.
text/markdown could be piped through a markdown to HTML converter, or displayed raw. Similarly you could add a text/markdown header to outgoing mail and just write .. markdown .. in your editor.
But most people wouldn't care, because most people don't even use console based mail clients any more. I gave up myself after having a virtual machine and reading mail over SSH for 20ish years. Nowadays? I pay for google workspace for myself, my virtual machine is long-since dead and I don't self-host anything myself in the cloud, just in my house where I have a NAS, backups, and stuff.