One of my users was claiming that emails are not getting delivered to a particular email address, I got the details from him and tracked the message from server end because user said there was no NDR.

This is the result from message tracking “420 4.2.0 RESOLVER.ADR.Ambiguous; ambiguous address”

The above error code indicates that the email address of recipient assigned to multiple recipients, hence transport server is not able to route the mail properly. If you go through your transport server event logs 9214 event will be there for the same.

In my case it was assigned to mailboxes, once I removed the mail address from second mailbox it was started working fine.

It can be assigned to mailboxes/contacts/groups/mail enabled public folders.

By default Exchange will not allow to add the same email address into multiple recipients, but if you use any tool to create Exchange recipients this can be happened.