Hi! Right now it doesn't support any external Email clients. Instead it has it's own IMAP protocol implementation, so if your email server supports IMAP protocol (most email servers support it), then you can set it up by opening Settings dialog and navigate to IMAP tab.
The default Email script comes with it only displays the number of unread emails, and the latest received email's sender and subject. The app was designed with modding in mind so if you like modding then you can open the script in data/scripts/satania/menu/Email.evil and modify it to your likeness. See Help -> Scripting APIs for reference.