I recently had to find a way to share our Exchange 2k3 email address
space between two AD forests as part of a merger. One method I tried
was making our Exchange Org non-authoritive for the namespace and to
forward mail to the second forest.

For various reasons I found a simpler and tidier method by creating
contacts, so I reversed the changes I had made previously.

However (and here's the problem!), when I changed my Exchange server
recipient settings back to being authoritive for the namespace, it has
now stopped generating NDRs for invalid addresses. I can see the
messages being sent back to the Outgoing SMTP connector and never going
any further.

Can anyone help with resolving this? I have confirmed that in all
recipient policies (incl Default) that the checkbox is ticked for 'This
Exchange Org  is responsible for mail delivery to this address' for our
namespace. Exchange services have all been restarted etc

Email rejection
Antivirus and Spam
Back-end Server
Routing Groups?
Automatically purge deleted items?