NDR is a bounce message, also called as Non Delivery Report/Receipt. It is a failed Delivery Status Notification (DSN), a Non-Delivery Notification(NDN) or simply a bounce.
On your mail relay usage log, you may find emails sending with blank from email address. In this case, you must have used our mail relay server as Smarthost ( or mail gateway) for your Exchange Server. When NDR is enabled on your Exchange Server, an non-existing email to the server will trig NDR reply to the sender's email address notifying the sender that the email address does not exist.
Spamming programs simply guess the common email addresses under your domain and it will generate a lot of NDR on your server when your domain got hit by Spam emails.
To avoid NDR mail relay usage, it is recommended that you turn of NDR on your exchange server. Click the following links to know how to turn off NDR on Exchange 2000 and 2003 .
Please click this link to know more about NDR at Wikipedia.