Zimbra

Are you looking for something a bit nicer than a traditional Unix IMAP server? Fancy (shared) Calendering, cool AJAX implementation, LDAP address book? SSL setup is a synch.

Then look no further. Zimbra rocks. Basically, it builds on some top quality Open Source software (Postfix, OpenLDAP, Tomcat, SpamAssassin, clamav, DSPAM, MySQL, ... ) and makes it all work well together. Not just well, but really well. There's an Open Source edition and a pay-for edition (which adds Outlook connectivity, and other stuff). The Pay-for version is still relatively cheap (compared with the competition) and could be worth the extra depending on your needs. You get all the required software bundled together in guaranteed working versions.

There's also a good community of users (there's a wiki and active forums). It's really easy to install (so that'll save you some more money). It's certified on Red Hat Enterprise Linux and can be bought from Red Hat Exchange if you're that way inclined. If you want to try it there's a VMware appliance you can try it on. (That's how I demoed it).

You can't afford not to try it.