I still use Mutt, combined with offlineimap (due to exchange mail server), w3m for for converting mails from HTML, using markdown to write email, and pandoc to format back to HTML... Works like a charm!
Did not find any other mail client (although tried mailspring etc) with such powerfull customizations, nothing even close to Mutt.
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
