Recently when in shift, I noticed that the scom alerts indicated more than 200 messages in queue of our transport servers. Upon checking the queue viewer I could see messages being piled up in the queue with the error 420 4.2.0 RESOLVER.ADR.Ambiguous; ambiguous address.
As the error says, the messages were getting piled up due to a duplicate smtp address. The event viewer log revealed the following:
The logs clearly says another AD object is assigned with the duplicate email address and the next thing to do is to find out the object and delete it.
So, you can search for the email address in Active Directory Users and Computers console in the custom search section with the “proxy address” attribute.
Once you find the AD object, delete the ambiguous address from the proxy address attribute of the other AD object.
Check and confirm that the issue has been resolved.