For some unknown reason our exchange 2003 server has stopped sending NDR's when an email is sent externally to an address which does not exist, but does if an email is sent internally to an address which does not exist, i.e.
Send an email to email@example.com (an address which does not exist internally) and exchange reports a NDR
Send an email to firstname.lastname@example.org (an address which does not exist externally) and exchange does not report a NDR.
I have checked that no filters are applied in Global Settings > Message Delivery. I have also set it to send a copy of any NDR's to my account and nothing comes through when sending an email to an invalid external address. Allow non-delivery reports is also ticked in Global Settings > Internet message format
The server is up to date with all updates, etc.
Hoping you can help