Delivery Status Notification (Relay)

When one of our users sends an email outside of our system with a delivery status notification requested, they immediately receive a DSN containing their entire original email as an attachment. The message reads:

Delivery Status Notification (Relay)

This is an automatically generated Delivery Status Notification.

Your message has been successfully relayed to the following recipients, but the requested delivery status notifications may not be generated by the destination.

We still want DSNs for problem messages but I’d really like to turn this immediate DSN that doesn’t provide any needed information off. Anyone have any suggestions?