It's critical when you ask questions here that you give us enough information to help -- what version of Exchange are you running, what kind of network, how does your server connect to the internet, etc. I recommend you do a search at http://support.microsoft.com for some combination of Exchange+NDR terms, there appears to be plenty of articles available. I'll just note that you might also look at Exchange Service Manager (ESM), drill down to your server name, protocols, SMTP, STMP virtual server, Properties, Messages -- you may find some things of interest there.

It's critical when you ask questions here that you give us enough information to help -- what version of Exchange are you running, what kind of network, how does your server connect to the internet, etc. I recommend you do a search at http://support.microsoft.com for some combination of Exchange+NDR terms, there appears to be plenty of articles available. I'll just note that you might also look at Exchange Service Manager (ESM), drill down to your server name, protocols, SMTP, STMP virtual server, Properties, Messages -- you may find some things of interest there.