Another Choice - Kolab
Another GPL choice is Kolab, kolab.org
I have had it running for several years without a hitch. It is close on the backend... but not as functional as Zimbra on the front end. I have installed and configured the products stated above, and others for several years.
Having already sipped the Kool Aid with Community Edition of Zimbra I just upgraded to 5.0.2 tonight.
The MS hostile takeover bid could take years to unfold.... In the meantime there is still time to make a choice.
One thing about Zimbra is the relative ease of leaving if you want.... taking most of your data with you to another product. It is hard to give up such a neato product though.
Keep your eyes and ears open!
Mark Street, D.C., RHCE
Chief Technology Officer
Alliance Medical Center