[MM3-users] Re: when on particular person sends a message to the list dozens of OTHER users have their bounce score incremented

See my results posted as a response to Odhiambo Washington above. However for thread completion (and to help others in the future), the answer to your question is:

The DSNs vary based on which provider responded (ie. verizon.net, sbcglocal.net, navy.mil, me.com, hotmail.com, cox.net, etc.), but one example (that very much supports above conclusion) is:

Diagnostic-Code: smtp;550 5.7.509 Access denied, sending domain r1de.net does not pass DMARC verification and has a DMARC policy of reject.