[Mailman-Users] rejecting messages *to* non-members

[glen martin]

Apologies if this is a FAQ ... yadda yadda ... searched wiki, list 
archive, FAQs, even the documentation :)

I've got some rude users who occasionally cc non-list members on their 
messages to a restricted list that is actually intended to be private 
(membership by invitation only).  Then, as you might expect, when the 
non-member replies, that message is rejected or held for approval.  This 
doesn't seem friendly.

The root cause, though, is my users.  I'd therefore like to train them 
:) by rejecting their posts that copy non-members.

A natural spot for such an option would seem to be in Recipient filters, 
but I don't see anything there, nor in the rest of the options for that 
matter.
The mailing list discussions around non-members seem to have been 
dominated by discussion of messages from them, not to them.

Is there such an option, to reject messages that are {to,cc} 
non-members? Or suggestions?

Apologies again if this is obvious or a FAQ that I've somehow missed.

thanks

glen