It seems mailman, by default, spoofs its outgoing message with the "from" address of the person who posted the message. The envelope mail-from is not spoofed, but rather the P2 FROM. Is this configurable to d a send-on-behalf or simply change to some other static address?
A bit old question, but here some suggestion solution.
I will try to explain how the default mailman behavior.
Email from email@example.com was sent to firstname.lastname@example.org (a mailman list). Both envelope sender and email@example.com have value "firstname.lastname@example.org".
As mailman receive it, it rewrites envelope sender to email@example.com. Now mailman forward it to other members with envelope sender: firstname.lastname@example.org and From header: email@example.com.
The problem is, as April 2014 the Big Email Provider Yahoo activates DMARC in their domains. As the result, if DMARC-aware mail server spot the Yahoo address in From header but it wasn't sent from Yahoo server, the server will reject it.
Mailman developer adding fix in Mailman version 2.1.16. It now has option from_is_list