I am surprised not to see Zimbra shown in the results especially when they have recently hit 40+ million paid mailboxes ? http://www.zimbrablog.com/blog/archives/2009/03/zimbra-turns-40%E2%80%A6-million-that-is.html

Next time you should include Zimbra in that survey - I've been blown away by their open source server in everything from features to performance & scalability - and that's not even their paid product.