A mail/news client written in Java, originally developed as part of the Javagator project at Netscape. This project was cancelled, but the source to these portions was released under the MPL.
It is currently usable, if buggy and incomplete, with support for multiple identities and reading newsgroups, as well as the basic functionality of sending and recieving e-mail. Development is slow, in fact at the time of writing there have been no CVS checkins in the last month.
www.mozilla.org/projects/grendel/