Problem

How do I reject incoming email for unknown users in MS Exchange 2003?

Solution

This solution is for Exchange Servers 2003. It is assumed your Exchange system is already setup and configured correctly

1. Start the Exchange System Manager
2. Select the properties of "Message Delivery" which can be found under the "Global Settings" folder
3. Select the "Recipient Filtering" TAB
4. Tick the check box labelled "Filter recipients who are not in the Directory" and select OK.

This solution will be prevent backscatter and stop your exchange server processing NDR's (Non-delivery reports) for unknown users.