We had an Exchange 2007 user report that she was sporadically unable to receive e-mail.  When we began looking into the problem, any bounced messages were being returned with 550 5.1.4 RESOLVER.ADR.Ambiguous; ambiguous address as the error code, and the error was being generated by our Exchange 2007 HTS.  There is very little documentation covering this particular error, so we were left scratching our heads for some time.  It quickly became clear that the "ambiguous address" was coming from a duplicate SMTP address assignment somewhere in Active Directory, but we had looked through all SMTP addresses on our Exchange 2007 accounts and could not find the duplicate assignment.  We also searched through Mail Contact objects, with the same result.

The following PowerShell command provided the right level of illumination: $AdminSessionADSettings.ViewEntireForest=$true

We then executed the following command to find the duplicate object: Get-MailContact *lastname*

When we changed our Active Directory visibility setting from within the Exchange Management Shell, we were able to find a contact object in one of our child domains that was setup by an Exchange 2003 administrator to forward messages back to our Exchange 2007 mailbox.  However that contact object had been aimed at an SMTP address assigned to the Exchange 2007 mailbox, and thus Ambiguous Name Resolution (ANR) failed to differentiate between the child domain contact object, and our Exchange 2007 mailbox.  We are unsure at this time why the child domain administrator was able to duplicately assign our "taken" SMTP address to a contact object in the first place, however removing the offending contact object corrected the problem.