If you want a groupware server to replace Exchange, with shared e-mail, shared contacts, shared calendars, etc, then have a look at Zimbra
. There's an open-source version that uses custom code to glue together Postfix, Amavisd-new, ClamAV, SpamAssassin, Cyrus IMAP, OpenLDAP, MySQL, and a custom AJAX web interface. It's really quite slick, and scalable.
Of all the ones we've tried (Merak, OpenXchange, Scalix, FirstClass, Kolab, and a few others), it's the best combination of easy-of-use, ease-of-administration, features, and compatibility. It's also one of the few that uses all open standards for accessing the data (SMTP, IMAP, SOAP, REST, LDAP, CalDAV, WebDAV, iCal, HTTP, and so on).
There's also a very easy upgrade path to their Network Edition, which includes over-the-air sync for blackberries and activesync clients, and a connector for full outlook compat (makes the zimbra server look like an exchange server).
The only downside is that they don't officially support FreeBSD.