Source-code can be found here.
Seems pretty cool, although the UI design is a bit too Microsoftish for my taste. Seems to support pgp encryption and CalDAV/CardDav sync.
Last release is a bit older, but the main developer’s blog has some interesting details on new features in the development version.
Honestly, aerc is all you need. I’ve been using it for a while and it’s been great.
Since we’re talking alternatives, at least compare it to a graphical one, namely astroid mail 😅