Mailman Problem URGENT!

A client of mine (a PR agency) has Mailman for all of their media distrubution lists. Today there was a big problem and somehow people subscribed to a list somehow sent mail out to it, even though the list is set to only allow mail to be sent from 5 designated addresses. A lot of people were not happy, thus they kept replying and thus more mail was getting sent out through the list.

I went through and checked the list settings, and I'm 99% sure I have things right. I have 5 admin email addresses and set replies to go to the poster. Am I missing something?

I set-up a test list, using all of the same settings as the other lists, and was able to send mail to the test list from an address subscribed to it even though it is not an admin email address. I replied to it and that mail got through, too. When trying to send from an address not subscribed and not an admin, it was rejected - which is good.

Can someone tell me if I am missing something? I need to set the lists so ONLY the admin addresses can send to the lists and no one else.

Thanks!