These messages are generated by SmarterMail when a message was received with a request for a Delivery Status Notification (DSN), but the server that SmarterMail passed the message to did not support generating DSN. This is determined based on the capabilities reported by the other server during the SMTP session.

You can turn off support for DSN messages by logging in as a system administrator and going to Settings > Protocols and disabling the "Enable Delivery Status Notifications (DSN)" option on the SMTP In card. Disabling this option will prevent these messages from being generated, but it will also prevent your gateway from relaying DSN requests to the destination server.