[Mailman-Users] how did spam message get through a moderated list?

[Aleksandr Miroslav]

Recently a spam message came in to four lists I administer. The email
was writtenlike this: firstmlast <user at example.com>.

I recognized the real name, because First M. Last, is someone I know
and is subscribed to all the lists. However, the "user at example.com"
was totally foreign. So spam.

Three of the lists automatically discarded the email, as I have
configured them to do, and sent me an notification about it. But the
fourth list sent it through, even though user at example.com was not
subscribed to the list, and I have generic_nonmember_action set to
Discard.

For the life of me, I cannot figure out how/why this email got
through. I have set the list to emergency moderation for now, but I'd
like to know how it got through in the first place.

I did a config dump and vimdiff across the 3 lists, apart from the
list names, signatures, and two minor fields (max_num_recipients, and
admin_member_chunksize), they are all identical.

In, /usr/local/mailman/logs/post, I see this for the message:

Jun 26 03:48:40 2019 (1052) post to listname from user at example.com,
size=6065, message-id
<xxxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx at xxxxx-xxxxxxxx>, success

For the other 3 lists, I see this in /usr/local/mailman/logs/vette:

Jun 26 03:48:31 2019 (1050) Message discarded, msgid:
<xxxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx at xxxxx-xxxxxxxx>'
        list: list1,
        handler: Moderate
Jun 26 03:48:51 2019 (1050) Message discarded, msgid:
<xxxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx at xxxxx-xxxxxxxx>'
        list: list2,
        handler: Moderate
Jun 26 03:50:22 2019 (1050) Message discarded, msgid:
<xxxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx at xxxxx-xxxxxxxx>'
        list: list3,
        handler: Moderate

Does anyone know why the message to the fourth list went through?