Understanding non-delivery reports
NDR or Non Delivery Reports are potentially a great way of telling a user that they made a typo with an email address and that the email could not be delivered. In a 1999 world, this would be fine.
However we are in a world now where email servers are flooded with spam, and lots of it, replying to every junk email that is hitting all the imaginary email addresses on your server (support@, admin@ help@ etc) is not a good idea, and it causes what is known as backscatter. To avoid this back scatter of invalid email delivery from your server it is recommended that you turn off your Non Delivery Reports NDR.
To do this in Exchange 2003 you need to perform the folloing:
1. Open Exchange System Manager 
2. Expand Global Setting and Click on Internet Message Formats
3. In the right hand pane, double click the “Default” name or whatever domain you have configured.
4. Click on the Advanced Tab and uncheck the option for Allow non-delivery reports
5. Ok and you are done!
