So far we have decided on an installer framework similar to that used by IPCop, a bootable CD-ROM image which installs a Linux-based firewall. The core mail system will be qmail, with several patches to add the functionality required for several features including virus scanning, spam filtering, and the ability to handle mail for multiple domains.
There is also a mailing list for people interested in the project, either as developers, testers, documentation writers, or end-users with suggestions about what kinds of features you would like to see.