[Mailman-Users] Strange bounce messages - why is it allowed through and how do I find out?

[Lars Bungum]

I'm running a list of about 500 people, and have encountered a strange
problem lately, namely that bounce messages from an address

"dummy at not.exist" are being sent to the list.  The subject is like this:

Autoreply: {digest-subject}

and body like this:
"This email adress do not exist"

so it appears to be bouncing the digest.  The address does not exist,
and obviously is not a subscribed to the list.  And the posting policy
is subscribers only.  A lot of messages are being held for moderation
because the posters are not members (spam).  Still this one comes
through. 

In the mailman/logs/post log file it just says that dummy at not.exist
posted to the list at this or that time.

I'm both bewildered and estranged - what is this?  And how do I find out
why it is allowed to post to the list?


-- 
--lars