Exchange Server 2007 Rejecting Unknown Recipients

Start up the Exchange Management Shell
cd “C:\Program Files\Microsoft\Exchange Server\Scripts”

Open up Start / Administrative Tools / Services.
Restart the “Microsoft Exchange Transport” service.

Start up the Exchange Management Console.
In Organization Configuration / Hub Transport, there is now an “Anti-spam” tab.
Right-click on “Recipient Filtering” and choose “Properties”.
In the “Blocked Recipients” tab, tick the tick box “Block messages sent to recipients not listed in the Global Address List”.
Click “Okay”.

You may well want to disable some of the anti-spam tests if you do not intend to use them.