I'm glad to have recently checked out Zimbra, an enterprise level email hosting and serving solution that competes with Microsoft Exchange. Frankly it looks a heck of a lot better than Microsoft Exchange on the cost factor, as well as the fact that it leverages, interacts, and can be integrated with open source email projects. For example, Zimbra uses Postfix (I LOVE Postfix), and it can use MySQL, Postgres, or OpenLDAP, among others.
I wish I had been exposed to Zimbra prior to migrating one of my customers to Google Apps for Business with GMail and the supposed support for Outlook. That was a total disappointment.
While I've only seen a demo of Zimbra so far, like the title says: Zimbra Looks Quite Nice.
Zimbra is brought to us by the same folks who bring us vSphere: VMWware. Surprising, huh? I think so, anyway.