[Exim] Exim as transparent "proxy"?

Hi folks!

Here's a bit of a wierd one, perhaps. I didn't find anything about it in
the docs or in the FAQ so I thought I'd ask here.

Is it possible to set up exim in conjunction with ip diverting or
policy routing to intercept SMTP connections/sessions that
attempt to leave a network? Something similar to ye olde
transparent proxy for HTTP?

Reason I ask is that I need to find a way to allow all our internal
mailservers to deliver to each other directly using DNS lookups
rather than forcing them to use a relay yet still have the relay
systems (exim) handle deliveries to the larger Internet. Visiting
each mailserver and hardcoding lists of domains for which to use
the relay (or not) isn't an option owing to the sheer number of
mailservers involved. NAT has already been considered and
rejected.

If anyone has done this or has any ideas, I'd love to hear all the
gory details. :)


Regards,

- Sean.