Hi

In my opinion, open-xchange[1] is a fairly good alternative to
Microsoft's Exchange server. I remember that there were a few
issues, but cannot recall - in the end, we have taken Microsoft's
product (since we are in the Microsoft Partner program[2] - might
be an option for your company?)


Good luck

Cheers


[1] http://mirror.open-xchange.org/ox/EN/community/
[2] http://partner.microsoft.com