I found Markdown Here. That is groovy, I guess. But it isn't what I want from an email client. I don't want to send (or read) HTML. But I do want to render Markdown, and style it the way I want. My email needs are so basic, and Thunderbird is starting to mess up for me for some reason.
If there were a replacement that could render Markdown without converting it to HTML, and plug in to GPG, that would be great. Although encrypted messages are so rare for me, I'd take it without it, since I can always use other tools for that.
Addendum: I decided to try using mutt for a while. It solves a lot of my frustrations in the variety of HTML messages I get from people. ^_^