Gnus on Emacs was very good with IMAP for me, where sources were Fastmail and Gmail. Encryption and signing works, but I haven't used it in email that much, so I can't comment.
Gnome's Evolution was good too, IIRC. There are also Mutt and Pine, but they are terminal clients, so IDK how they handle inline images, tho mutt was OK with html.
I use notmuch[1] in emacs, fetchmail to get email, and local MTA set up to deliver to my maildir.
HTML email is rendered with w3m which does a decent job.
Searching my local email with notmuch is blazing fast and works even better than Gmail's searching (IMO).
Certainly not everyone's cup of tea but if you like emacs otherwise, it's a great alternative to Gnus. Gnus is originally and primarily a newsreader and treating email like news works in some ways but fails in others. Also searching in Gnus is something I never really got to work well.
I'm using a rmail+m{pop,smtp}+mairix+spamassassin approach nowadays (My email provider's spam filtering is crap, so I do it myself, with a whitelist from my BBDB). Notmuch always seemed interesting but I couldn't dive in yet. I have some free time this week, so I'll try. If your configuration is public (i.e. on github), would you mind sharing a link?
Gnome's Evolution was good too, IIRC. There are also Mutt and Pine, but they are terminal clients, so IDK how they handle inline images, tho mutt was OK with html.