We have been experiencing a strange anomaly. We are running a small-ish list. When a particular subscriber to the list sends an email to the list, a couple of dozen (or more) of the recipient's of the message end up with their bounce scores incrementing. This particular subscriber uses about three different email addresses when he sends his messages and every one of them causes this behavior. There is another subscriber who also causes this behavior when he sends an email to the list, but only one of the two email addresses he sends from causes this behavior. There is a third person who causes this behavior every time. There are dozens of other subscribes who don't generate this behavior when they send to the list.
I can't figure out why this is occurring. I *assume* that Mailman re-writes the headers to add DKIM, etc., before sending them to the list recipients but it sure seems like something in the header from the original sender is ending up in the final header (or something is LACKING) which is not liked by the recipient's MTA's. It doesn't seem like a subscriber sending an email to the list should be able to cause this behavior - but I'm no expert.
The recipients appear to be using many different mail platforms (ie. their addresses are not all, or even mostly, @google.com or some other domain; the recipients appear to be a pretty good cross section of platforms).
This has been going on all year (maybe longer). Any help or troubleshooting suggestions would be greatly appreciated.