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.
Project Cyrus
Cyrus IMAP Server Installation FAQ
eMailserver Kompendium (German)
Configuring the IMAP Server
Cyrus System Administration with cyradm
(O'Reilly)
Hosting email for virtual domains using Postfix and Cyrus
Postfix-Cyrus-Web-cyradm-HOWTO [Luc de Louw' HowTo]