Hello Scott,
The bouncing is not the media-motion list servers fault, it’s Yahoo who implemented the DMARC policy, so when a Yahoo subscriber on the list posts to the list, all users from Yahoo, Verizon and Gmail get bounced.
A solution to stopping the bounce messages to be sent out is to set the server to never unsubscribe because of bounced mail, but then all Yahoo, Verizon and Gmail subscribers would stop getting mail from this list.
Almost all mail list servers around the world are suffering from this, and I’m surfing the internet daily to find a solution to this problem.