Having a real scripting language allows a lot of useful features, at times using mutt felt a little bit too constrained, although it has a lot of features and is very very flexible in its own right:
~/.muttrc: alternative_order text/plain
One concern is that I'm letting through all web bugs and, perhaps, outright malicious HTML. If anyone has a solution for that, something like filtering the HTML, I'd love to hear it.
No, but HTML emails, 99% of them, are just plain text in an HTML markup. You won't miss garish colors or font choices.
As for embedded images etc, you can still view them with plain text mail clients (as attachments).
If I really end up having to read an HTML email those can be displayed "inline" using a text browser such as w3m. If the HTML is mostly text it works pretty well, for instance: https://svkt.org/~simias/mutt-html.png
If however it's image or css-heavy and doesn't work well in text-only form there's still the option to tell mutt to open the email in your web browser. Of course in this case there are security implications so it's probably best not to do it from untrusted sources.
Recently it landed full Lua scripting support as well. Guess you're not the only one that was looking for more powerful configuration within mutt.
I'd be very happy to have more feedback and help to extend further that API and have some great scripts happening!
For example you might have a folder of "backup results", you'd want to mark all that have a subject of "Backup OK" as read, and flag the ones that have a subject of "Backup FAILED" as "important". Those are the kind of tasks I automate with my lua-based mail-client.
You can get more interesting and say "Keep only the most recent 100 messages in folder XXX" which applies whenever you either launch the client, or open a given folder too.
https://www.neomutt.org/changes/user