One or more messages have failed delivery due to DNS response 'non-existent domain'. This will occur if one of the DNS servers ...

One or more messages have failed delivery due to DNS response 'non-existent domain'.  This will occur if one of the DNS servers in use does not know how to resolve the recipient domain.  Check the recipient domain and ensure it is correct.  If you believe this response was received in error, the connectivity log should be enabled to determine which DNS server is providing the incorrect response. The connectivity log can be enabled within the Exchange Management Shell by issuing the command "Set-TransportServer -Identity '{2}' -ConnectivityLogEnabled:$TRUE". The log is located in directory {3}