I have to be skeptical of any email client that's not integrated in to Emacs.
What about integrating emacs into aerc? You can set your EDITOR to emacs and replace aerc's default keybindings with a more emacs-inspired set. Send me your binds.conf and I'll add it to contrib in case anyone else wants an emacs-esque setup, too!
I appreciate the suggestion, but unfortunately that's not even remotely adequate.
By integration I mean exposing every single function and UI element of the client to Emacs, so that they can be bound, wrapped and extended as needed. Ideally, these functions would be written in eLisp, so they themselves could be modified and customized to taste while leveraging the enormous Emacs eLisp ecosystem.
Emacs email clients have this sort of integration, and it is what gives them tremendous flexibility and power.