@No Dice Really? You can buy Exchange for $12.67/user per year?
For 50 users as an example
$28/user per year for 50 or more
$500/50 users per year
Total = $38/user per year
Exchange 2007 Standard Server
Total = $80.98/user per year
Neither option gives you BES, both get WinMob, neither gives you support, both require OS but Zimbra can be on a secure OS that is cost effective if not free.
Not exactly apples to apples and with potential discounts for this that and volume, I still don't believe that you can legitimately get Exchange for a third of Zimbra. Even if you did, you'd still be using Exchange.
I suspect that you are using Outlook against a POP server and think you have Exchange.