Non-delivery reports have a very legitimate purpose and are used to notify senders of any errors that may have been encountered during message delivery. Such reports can help the sender find out if the email address no longer exists, if the remote mailbox is over quota or provide information detailing why the message was not delivered.

Unfortunately, spammers have started abusing this system to get around global white lists defined by mail server administrators. Most anti-spam software does not attempt to filter non-delivery reports for spam content. These two circumstances alone create a wonderful opportunity for spammers to relay their spam without being tracked or blacklisted. They spoof your email address and send the spam message to a non-existent email address on the target mail server. The target mail server produces an error and returns it to you along with the content of the spam message.

Non-delivery reports should be kept in place for error reporting, but if your email server becomes a target of a spammer, you should have the option of disabling it. Thankfully, in Exchange 2003 this process is simple:

Disable Non-delivery Reports

Open your Exchange Server Manger, usually by clicking Start > All Programs > Microsoft Exchange > System Manager.

Expand your Organization, select the Global Settings folder and then select the Internet Message Formats. Right click on the Default Internet Message Format and select properties. Click on the Advanced tab and clear the checkmark next to the “Allow Non-delivery Reports” as shown below:

Click on Apply to disable NDR's. Thats all!

While temporarily disabling NDR's to combat spam is acceptable, permanently removing them can disable crucial error reporting to legitimate users and email senders will not know if the message has reached you.

Read my other Exchange articles:

Publishing SenderID records for Exchange SP2 IMFv2
Enabling IMF 2 in Exchange 2003 SP2
Changing Exchange 2003 Store Database Limits
Exchange 2003 SP2 for SBS
Modifying the Outlook Web Access Login Page
Disabling NDR (non-delivery reports) on Exchange 2003
Setting up Exchange 2003 as an ATRN Client
Setting up Exchange 2003 as an ATRN Server