I have a network with Exchange 2010 on the root domain and a couple of child domains with a mix of Exchange 2010 and Exchange 2003 servers. One of the child domain Exchange 2010 servers does not communicate to another child domain Exchange 2010 server, although, it works the other way around. As it turns out, the users on one end cannot send emails to the users on the other end. Any thoughts?

The NDR message: #550 5.1.1 RESOLVER.ADR.RecipNotFound; not found ## This only shows up when sending to that particular child domain's Exchange users. Users on this child domain can send emails to the Exchange 2010 server users on the root domain.

Hi V3Bok, Please try to use Telnet to test communication first: Telnet to Port 25 to Test SMTP Communication http://support.microsoft.com/kb/153119Frank Wang TechNet Community Support