Check out Xmail, http://www.xmailserver.org been running one for years currently with 26 domains and just over 1800 accounts.
I am looking at building my first Linux email system. I very much would like to use Linux over windows/exchange for all the obvious reasons. The Servers need to host @1400 pop3 accounts. I was wondering if any of you could tell me if it is possible to cluster multiple mail servers for redundancy/performance. What distro / mail server software do you all recommend?