Project Cyrus builds the excellent Cyrus IMAP4 mail server. Right now it's the mail server providing the best integration with OpenGroupware.org, for example OGo can be used as a Sieve frontend for server side filtering and to configure Cyrus vacation messages.

The Cyrus mail server is a standard part of almost any GNU/Linux distribution and can be easily installed as an RPM or Debian package. Further, there are prepackaged configurations of Cyrus, like Kolab, which make installation even easier.