Issue: Postfix LMTP does not pass messages onto MM3:LMTP.
I don't understand why this is a Mailman problem. Seems like Postfix user lists would be a better venue to get expert help with this. Among other things, our most responsive Postfix guy is out of cell coverage for now.
I would post this To: the relevant Postfix list, Cc: this list, and set Reply-To and Mail-Followups-To to the Postfix list to get everybody who can contribute on the same page.
If it is a Mailman problem, you have my apologies and I'm listening.