Exchange 2003 SP2 will implement a new feature where by an administrator can control MAPI access to the Exchange 2003 server, on a per user basis. This feature will be implemented via the attribute named ProtocolSettings, on the user object in Active Directory.

Some of the access levels include:

• Full access
• Block all access
• Block online access (User not running Outlook/Exchange Cached mode)

To read more about this feature take a look at this article on  You Had Me At EHLO, The Exchange server team blog, here