On Thu, 2021-10-21 at 18:57 -0400, Konstantin Ryabitsev wrote:

thus making any message I touch pass DMARC verification.

Er… "thus making any message I *send* pass DMARC verification".

The Sender is the entity who sends the message. Who submits the message
to the mail system for transport. The Sender in this case is Mailman,
and the DKIM signature correctly passes verification.

All else is based on a fundamental misunderstanding of what the Sender:
and From: headers actually mean.