nmh, which despite its name is not very new! It is very nice, command‐line oriented and very scriptable (no curses interface). Unfortunately it is also very old—but at least it supports MIME.
For sending mail, well, I use OpenSMTPD as a relay to GMail, since it’s in base (on OpenBSD) and uses TLS for auth. I’d like to encrypt my email too, but haven’t gotten around to it yet…