I am especially looking at alternative after Google started charging for the use of GMail for small businesses (in spite of their promise to support up to 50 us

I am especially looking at alternative after Google started charging for the use of GMail for small businesses (in spite of their promise to support up to 50 users free). I am sure the alternative will not be as much polished, but I can roll my own postfix+roundcube and run it for 100+ users at < 100$ a year vs Google's $5000 a year! I don't need all the social shit Google throws in with an account, just mail and chat will do.