[Exim] Refusing mail

Hi!

I've looked through the FAQ, docs and tried looking through the list
archive for help on this but haven't turned anything up.

I run an exim box as part of a firewall setup for several sites.
Mailservers behind the firewall are a real mixture of Exchange,
Mercury, Groupwise... all kinds of things. The way things are
supposed to operate is that the internal mailservers are configured
to use the exim box as a "smart relay" for any address, intranet or
internet, that isn't local to themselves.

Unfortunately for me, some of the admins appear to be a few cards
short of a full deck and are blindly forwarding *ALL* mail to the
exim box, even if the recipient is one of their own users. I'm
desperately trying to find a way to reject messages of the type:

sender = fred@???
recipient = joe@???

.. yet still allow:

sender = fred@???
recipient = joe@???

... or:

sender = fred@???
recipient = joe@???

..., preferably during the SMTP dialogue so that I wouldn't have to
waste disk space and bandwidth accepting the whole message
first. An SMTP response something like "5xx Your admin is
incompetent - go and educate him with a clue-by-four." would
pretty much be first prize on this.

Does anyone have any ideas on how this might be accomplished,
assuming it's possible?


Regards,

- Sean.