[MM3-users] Handling of spam

I have a mailing list that is open to nonmembers. Sometimes we get spam on that list. I have a working spamassassin/amavis spam filter on the system.

Now comes the strange part: sometimes a spam message is put into a multi-part message and forwarded to the list owner (me). This multi-part message contains the message marked as spam, but is not marked as spam itself. The origin of those message is the bounces address of the list.

So my first question is: where can those messages be generated? I can provide the message header, but it would contain a lot of details of my server.

Cheers, Eggert