Re: It's a real shame.
Note I am in no way involved with the company, just a content (as in happy) user of the software: give Communigate Pro a look.
Years ago I was a paying user, running it on Solaris in a SOHO environment. If you ran it without paying, the worst they did was a attach a little "you are running this on a free version of CGP" into emails. Nowadays, it's free with full functionality for up to 5 users with quite fair support pricing for higher numbers.
It's a full enterprise communication suite, with email, IM, scheduling, VoIP(!), and a host of others that I don't even use in my little SOHO setup. Runs happily on any Linux distro I've thrown it on, also available on WIndows, IIRC. Very kind in its use of resources.
Works like a charm with Outlook clients, if you need to, as it comes with MAPI adapters.
I was going to use Zimbra but I had many of the concerns voices here, such as expected life of the product that was being bounced around like a hot potato. In addition the free version was crippled. Not the case with CGP.