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.
MyExchangeLync 
Today i came across similar issue.
I remembered that you have posted this in FB and gone through this which helped me to fix š
I’m glad it helped you š
Thanks,any idea why exchange 2013 does not send NDR for Ambiguous address?it uses to send in 2007/2010..is there a specific setting that need to be enabled for that specific NDR
NDR should be generated http://technet.microsoft.com/en-us/library/bb232118(v=exchg.150).aspx
are you getting the event?
yes,im migrating 2007 to 2013 and when sending from 2007 mailbox to ambigous.addrress ndr recieved immeditly however it does not when sending from 2013 they just get stuck in mbx submission queue.
other type of ndr are recieved from 2013 for internal and external but ambigous address is not being genrated