Im using markdown for writing email, and then on "send-message" I have macro that converts to html (pandoc), and sends an e-mail.

I agree with you, html mails can be usefull and people like them more than plain text (specially marketing,sales, etc).

I believe I have sweet spot with mutt, lot of custom configs being very productive, but not ignoring html . Sidenote, I use xombrero to view html mails if they are complex (inline images etc) - it is very fast and always shows mails correctly