I'm still constantly surprised that this feature isn't ubiquitously built into mail clients by default and users don't get a big 'enable encryption' button which automates sane defaults/manages the keychain transparently. It doesn't seem like such a complicated abstraction that it needs so much manual setup
I remember it being built-in to clients in the past. Certainly Evolution (Ximian, Novell, Gnome) did when I last used it half a decade ago. You could encrypt your message, sign it, and the other side would (if they had your public key) see a nice 'Signature verified' on the other side.
It was a fairly smooth workflow except for having to type in your passphrase in for everything.
