I am experimenting with a Linux box to completely replace all Windows back-end for an office. I've been fighting with Kolab on Kubuntu 6.10 for several days now, and I am also considering some other options but from what I've seen if I could just get Kolab to work correctly it will give the lowest cost and best support to Microsoft Outlook clients, plus it has a Horde plugin. I'm interested to see what other people who are setting up an Exchange server replacement use.