> For emails sent by an evil client that doesn't provide a plain text version, you can consider a tool that attempts to convert HTML to plain text (though I don't have a suggestion for this yet).
I use emacs to read my email, and w3m to render HTML email as text. It does a good job. Newer emacs includes its own web browser (eww) but I have not tried it for HTML email rendering since I'm happy with w3m.
Replies always go out in plain text (my preference) but if you want to send HTML there are ways to e.g. write your email in org-mode markup and have it converted to HTML when you send. But IMO if you want to send "rich" email then just use a client that does that natively.