Cyrus is a scalable IMAP4 mail server designed and implemented by the Andrew group at Carnegie Mellon University 
http://asg.web.cmu.edu/cyrus/. It runs on some pretty grunty hardware (http://asg.web.cmu.edu/cyrus/config.html), and serves a large campus, so it seems to work well. Certainly, they dont have problems.
Cyrus is, however, one of the most poorly documented pieces of software I've come across.
Redhat no longer support it out of the box, but there are contrib RPMs for the latest version available at http://home.teleport.ch/simix/Cyrus-imapd/
Also, see CyrusNotes...
