I have used qmail , sendmail and postfix.
I am not great with any of them, but the 1 I have had real good luck with is qmail. I suggest checking out
www.qmailrocks.org this site will do a step by step install for you from beginning to end including spamassisan, clamav, mysql-auth, imap ect.
If anybody has had any experience with installing Openexchange with Fedora Core 2 I would greatly appriciate any help. The java crap is killing me, not to mention its tested with postfix and I use qmail.
anyways good luck with your mail server!