Locate something in the rejection notice that you can filter on then add a global rule that deletes the rejection and breaks the cycle.  For instance, I have an expression filter that detects "*550 5.7.1 Backscatter DSNs not accepted*" and deletes those messages.  I don't specifically remember the service the generates this content though.

I have a MercuryC module using a third-party email relay service. I have a couple users who have their emails forwarding to an external (gmail) account.  

A problem surfaced when there was a rejection from the relay service one day.  A couple messages arrived for my forward-all-emails-to-gmail users, which MercuryC tried to forward to their gmail accounts.  The relay service rejected, which requeued the messages, PLUS added a delivery failure notice to these users to the queue - which it then attempted to forward to their gmail account, creating more failure noticed.  Long story short, 3 emails quickly ballooned to >8100 messages in the queue!  When the issue with the relay service was resolved, it ate up my 150 message per day limit in a matter of seconds.  I cleared the MERCURY\QUEUE folder, so problem solved. 

But is there a way to prevent this from happening again?  Ie, can I prevent delivery notification failures from being forwarded, or not generate delivery notification failures when the error on MercuryC is that the relay service is refusing the connection, not an endpoint-failure?