By default, an exchange 2007 installation does not contain a postmaster mailbox which catches all NDR’s. You have to create the mailbox manually and then configure it. This has to be done on every hub transport server in your organisation.
Step 1 is to create the postmaster mailbox. This only has to be done once:
Once this is done, you then need to tell the Hub transport server that this is the postmaster mailbox. To do this you use the cmdlet set-transportserver
set-transportserver exch2007mail –externalpostmasteraddress email@example.com
The above example sets the postmaster mailbox for the hub transport server exch2007mail to firstname.lastname@example.org. Once you have done this you can use the get-transportserver cmdlet to verify that it is correct.