In the past I understand that NDR can be generated from either sender or recipient server. To fight backscattering we used to disable NDR for the recipient's server and leave this to the sending server to do that with a smtp error reports from recipient's server. But with Exchange 2013 I am confused.... When I didn't NDR, the server neither reject the unknown user nor generate NDR (as configured).... since there's no error from the recipient server, sender server treats it delivered. Am I missing something here? Thanks in advance.