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mbsync/notmuch/Emacs.

I'm the happiest I've ever been with email using that combination.

I've explained the basics of my setup before:

(Granted, I'm a heavy Emacs user, so I'm predisposed to like writing emails in Emacs.)

I use something similar but offlineimap/notmuch/emacs, and it's the happiest I've been with email too.

I previously used elm, mutt, sup, and probably a few other clients, but none of them felt as configurable and complete as my current set-up.