[Mailman-Users] Unrecognized Bounce

More likely, what is happening is there is a list member whose mail is
sent directly or indirectly (after a forward of some kind) to the
server at tempest.organization.org. This mail is undeliverable by that
server so it is sending some kind of DSN indicating the delivery
problem back to the list-bounces address
<ascls-co-leaders-bounces at my_list.org> which is exactly what it is
supposed to do.

Mailman is unable to determine the bouncing address from the DSN which
is not surprising if it consists only of the four headers you quoted
with no other information in a body or attachment, so Mailman forwards
the unrecognized bounce to you.

Now it is clear from the bounce log that the message contained at least
a Message-Id: header in addition to the four quoted headers. What else
is in that message that your MUA is not showing to you? Can you view
the raw message source? If your MUA is the same Apple Mail you sent
this post with, open the message and select Message -> Raw Source from
the View menu and see if that gives you more information.


You prevent it by removing the offending address from the list. If the
above doesn't help find the address, you need to temporarily at least
set

VERP_DELIVERY_INTERVAL = 1

in mm_cfg.py and then restart Mailman. This will cause each message to
be sent individually to each user with envelope from
<ascls-co-leaders-bounces+user=example.com at my_list.org> where
user at example.com is the member address. Then, when it bounces, both
Mailman and you will know what address is bouncing.

Note that if you have one and only one list member whose address is in
the organization.org domain, you could try temporarily disabling
delivery to that member, and if that stops the notices, you've found
the undeliverable address.