A feature is available to disable MAPI program access to a computer that is running Exchange Server 2003
Though this feature that you can now add to Exchange 2003 SP2 sounds great, to me it looks half baked, seeing that it depends on you knowing the MAPI process name and such. Also why does it say "This feature requires the connecting program to use the Microsoft Office Outlook 2003 MAPI client version. "? I wonder.
I guess this feature will be much improved in Exchange 2003 SP3 and Exchange 2007 to provide a mini-application level firewall where you could decide which type of client can access Exchange from where.
For more information about thi hotfix: