restricting outgoing emails on qmail
 

I'm using qmail with smtp auth on my system. I'm planning to limit some of
my users to send emails outside my domain abc.com. Is there a patch or
qmail file that can do a restriction? Scenario is like these,
john@abc.comcan send an email to
peter@abc.com but cannot send an email to any other domain except only for
abc.com. But fee@abc.com can send to any domain.

Feel free to provide other solutions if you have similar setup. But I would prefer qmail since it was already setup