[Exim] <> not getting through

Hi there,
We have been using exim as our corporate mailserver(some 200 domains) for
some time now with great success and happyness :). We have now decieded to
make it our gateway server for our normal users (we are an isp). we have
been running it with relay_match_host_or_sender set and mail goes through
fine. we now wish to block mail with senders of domains that we don't
control coming through our nt based mailserver (please don't ask, the boss
just likes it for some reason), set up pop auth b4 smtp, then I removed
relay_match_host_or_sender and set sender_address_relay and the other
settings associated with it.
restart exim and everything is cool. however the ntmail server is sending
an email from say fred@??? to fred@??? it gets rejected,
which good, however the nt mailserver then can't deliver the reject message
to the hotmail.com user (which is one of our users using oe or the like,
with an incorrect setting) because exim can't verify <>, which is fair
enough. is there anyway around this but still incorporating the features
required. I tryed sender_verify_fixup and all kinds of options, to no
avail.
as I understand it setting up a router or director would be no good as that
is after verification.

could I use something like sender_verify_hosts and use the system filter or
is there a better way.
Any Ideas would be most greatfull.


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