[Exim] VPO's and mail relaying problem.

    I have a question regarding the setting up of sender verification.
 Our Exim box, version 4.04 on AIX 5.1, is set up as a relay for our
internal Exchange server which does VPO's for users.  What I would like
to do is verify the sender's address for all mail to the Exim box, and
bounce it unless the sender is *younglife.org.  This is necessary,
because our Exchange server is not capable of blocking a relay, and it
routes all of the outgoing mail to our Exim box for delivery.  Which is
then delivered by Exim because the Exchange server is listed in
relay_from_hosts.
    Any idea how I can accomplish the sender verification to eliminate
the relay through our Exim box via our Exchange server?  I have read
about sender_address_relay, but am not sure where to set it up in the
configuration file.
Thanks,