Our groupware requirements are mostly a shared calender, archived mail, and contact list. we use IMAP for email retrieval from our accounts (externally hosted for historical reasons), oh and it has to be self hosted without a maintenance contract (sorry igaware).
For an email client we are all using Outlook 2010 as it came with Office,
I installed and managed to get set up Zentyal Development Edition 4.2 (its free), with one outlook account (we have a very shared system) not using AD.
After adding the ‘exchange’ server to Outlook on four machines (and trying to add it on the fifth), I found some strange behaviour, it disconnects randomly; looking around I found this:
Not sure how much it helps but I have used Zarafa in the past and they do allow (or did back then) their product to be used free of charge for up to 3 outlook clients and after that you have to pay for licenses.
I am not familiar with Zentyal server but I would suspect the “first 3 users free” thing likely comes from the Zarafa component.
now a quick look on the appropriate page of the specifications very much does not show this as a limitation:
FFS, I chose Zentyal because it was supposed to play nicely!