How can I prevent users from setting up rules to automatically forward email messages to an external address?
In both Exchange Server 2003 and Exchange 2000 Server, you can block this behavior by using the Internet Message Formats object in the Global Settings node of Exchange System Manager (ESM). If you clear the Allow automatic forward check box on the Advanced tab of the Internet Message Formats object Default Properties dialog box, as Figure 2 shows, users won't be able to use rules to automatically forward messages. This restriction applies to all users in the Exchange organization. If you want to selectively let some users forward messages, see the Microsoft article "How to Override Blocked Auto-Forwarding for Select Users" (http://support.microsoft.com/?kbid=317652), which describes how to create an external contact object and allow forwarding to that object instead.
