[Mailman-Users] Reject *to* non-subscriber

[glen martin]

I've seen several posts about restricting non-subscribers from posting.  
This is a little different.

I run a private list where the list-members have decided they don't want 
non-subscribers on the threads at all.  But occasionally one will forget 
and cross-post. I'd like to bounce subscriber messages where a 
non-subscriber is anywhere on the receiver list.  Is there anything like 
this currently available or contemplated?

Thanks

glen