I ran into a problem this week where a user was sending email to an external email address and the message would leave outlook, but never arrive. Interestingly this user also could not send email to another internal user who was configured for forward to this same external address(via a contact in Active Directory). Other users were able to send messages both to the internal user (and have them forward) as well as directly to the external email address without issue.
I did some google-ing on the 420 4.2.0 RESOLVER.ADR.Ambiguous; ambiguous address and kept coming up with results that wanted me to check for duplicate proxy addresses in exchange. See this article for how to figure that out: here. After searching I was unable to find the duplicate entry, so I moved on.
What I ended up doing was deleting the external contact, re-creating it, re-set up forwarding for the internal user in question and then tested again. As soon as that contact was deleted the submission queue started to empty, and inital user who reported the problem was now able to send to both addresses, just like the other internal users.
Not an elgant fix, but I figured I’d share it anyway due to the low number of results that I found on google when searching for a resolution.