This is notes from a recent troubleshooting case. The client received the error: 420 4.2.0 RESOLVER.ADR.Ambiguous; ambiguous address and a lot of messages ended up in the Submission queue with this error.
The error came after running GALsync. We have two Exchange 2010 forests and was running GALsync between the two. Forest A (contoso) was using one address space (contoso.dk) and Forest B (nwtraders) was using several address spaces (nwtraders.dk, litware.dk etc).
Everything working fine between contoso.dk and nwtraders.dk, the two main address spaces, but the problem with 420 4.2.0 RESOLVER.ADR.Ambiguous; ambiguous address, came when trying to send to address space litware.dk from Forest A.
The error was caused by duplicate SMTP proxy addresses.
I have seen it before where GALsync created a contact during a sync to the target forest even though the user was already migrated to the target forest. Then we have the both a mail contact and mail user had the same SMTP address and messages to this user will be queued.
In this case the issue was quite similar, although the client had created the extra mail user themselves and when GALSync created the mail contact, the problem came.
The solution was to clean up in the proxy addresses of the two objects (mail user and mail contact) and good way of checking this is using ADSI Edit to see and confirm the proxy addresses of the objects.