I haven't researched Notmuch, but I've heard some interesting things about it. I used to use Mutt back in the day. Whatever I use would have to have Org integration, though.

Tbh, I just haven't researched other things and I haven't had the time. But Gnus does seem to fit well how I organize my mail: I subscribe to a lot of mailing lists, each of which are filtered into their own folders via Sieve scripts, before they touch my MUA. I also organize normal mail similarly.