Internal email only...?

With apologies to Mr. Sam -- I realize I may need to put this question
to either a vpopmail list or a qmail list, in the end, but I gotta
start somewhere. :-)

I have qmail (netqmail 1.04), vpopmail 5.3.30, and SqWebMail 3.6.2
serving a double handful of virtual-host domains on a particular
RH 8 server.

I need now to set up one domain on the same server, as an "internal
only" mail center, one that will allow users of the domain to send
messages to each other but will _disallow_ sending mail outward to
other Internet domains.

Any tips, please, on how to accomplish that?

Thanks kindly,

-- 

 -- Jeff --   <http://www.wellnow.com>

 "There's nothing left in the world to prove.  All that's worth doing
  is to love one another, using whatever means are available to serve."