Post To List But Do Not Receive Emails

Have looked at the documentation for EZMLM and can't find anything that 
will actually do what I need it to do.

DomainA has a mailing list
Only a few people in domaina should receive emails from this list.
but everyone with-in domaina should be able to send to this list.
Company does not want the list opened up to the outside world.  Wants it 
so only domaina can post to the list and no other domain can.

Hope this makes sense.

Is there something I am missing in the configuration of EZMLM?

Rob G