[Mailman-Users] List member seen as non-member by Mailman?

[Ulf Dunkel]

I am running Mailman 2.1.16 on an up-to-date Ubuntu Linux machine.

On one of the lists, I frequently get admin messages to accept messages
to the list, sent by non-members. But when I check them, their email
addresses are definitely in the list.

None of the members are set to be moderated, nor do I find any other
restrictions.
Is there anything I can have overseen so that Mailman doesn't recognize
these list members?

Thank you in advance,
---Ulf Dunkel