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.
BTW, I just tried it out, and I couldn’t connect to my email server, nor for cladav/cardav. Weird enough it doesn’t allow almost any settings, for example, I couldn’t configure the ports for the services. And preferences doesn’t allows changing anything.
It might be it’s too early to give it a try, or that’s the plan, inhibiting the user to control anything…