Problem:

How do I reject incoming email for unknown users in MS Exchange 2007 & 2010?

Solution:

This solution is for Exchange Servers 2007 and 2010 and using the anti-spam agent for exchange. This is an available feature in the Edge Transport Server. However if you don't have Edge Transport Server it is possible to install the anti-spam agent in the Hub Transport Server. Instructions for this can be found here.

1. Start the Exchange Management Shell (EMS)
2. Type the following:
   
   Set-RecipientFilterConfig -RecipientValidationEnabled $True

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