So close on mutt! :)
I have it set up so that it autoconverts all HTML messages to plain text as best as it can. If it's not good enough, I have a macro set up to launch the HTML version in Firefox so it's usable. (None of the images come through, which is potentially a feature.)
I did look into writing HTML mail with mutt, and it's even uglier than reading. The gist of it is to basically have a wrapper script that launches some kind of HTML editor, then builds the multipart message (maybe autoconverting HTML to text so you can have both) and headers, then launches mutt -H email.txt
to prepare to send it. If it looks good, send it from Mutt as normal. I don't know how well this would work with attached inline images, but it sounds potentially quite painful.
But I don't regularly send HTML messages, so I haven't bothered with that route. I'd just bring up TB if I had to.
(I can say, for me, since I went back to mutt, I'm happier with email than I've been for decades. And my RAM is happier, too. But I probably spent 20 hours configuring it. And everyone probably hates my preformatted text. They get back at me by sending 30 MB HTML-only mails. ๐คฃ)
I'll have to post it all somewhere sometime. None of my passwords are in there, but some of my account names are.