There didn't seem to be an answer, posted with Mailman FAQ 5.6 (see below).
I am having the same problem...I have bounce processing turned on, but I'm
not getting any bounce back message back to me (the list administrator when
I send to a non-existent/invalid email. What do I need to do to get bounce
messages sent to the list administrator?
5.6. Cannot Find/Receive Bounce Messages
I am trying to review the bounce messages - as with the new SPAM blockers if
you are not on the person's "approved" list you email may not go through. I
cannot seem to get the bounce message that Mailman cannot process - with or
without Mailman bounce processing on. I have tried setting to send the
messages to the list owner, and have set the posting by non-members to be
held and forwarded. I get one or two messages sent to the list admin, but
when I turn off processing and purposely send to a non-existent address - I
receive nothing back in either the master account for the domain, the list
admin account or the posters account.
What am I doing wrong?
Thanks in advance