I thought some people here would be interested: Recently SOGo added support for OpenChange, which means Outlook can natively access it via MAPI the same way as Exchange. Everything is open source, and the great thing about SOGo is that it uses existing IMAP server for mails rather than reimplementing yet another one itself. They seem to prefer Cyrus, but apparently Dovecot should work too. For non-mail (calendar, etc) it uses SQL, but I heard it's not too difficult to replace that either if someone would prefer some NoSQL database or something.
http://www.sogo.nu/ http://www.openchange.org/
