[exim] Unusual routing question [OT]

I'm wondering how to do something tricky, it's perhaps slightly off topic.

I'd like to use something sort of like xinetd where it would listed on
connections on port 25 and route them to exim (or any other MTA someone
might be using) IF the connection is from certain host names. xinetd
will do that.

BUT - this is the tricky part.

I want to route the non-matching traffic to a different program. So some
port 25 connections go to exim, and some don't depending on the RDNS of
the IP connecting.

I need it to be simple as well so that Linux admins that aren't
programmers can figure it out using fairly standard tools.

Thanks in advance.

BTW - the idea is that my customers install it on their server. email
from *.junkemailfilter.com is routed to their MTA. But connections that
don't match are routed perhaps back to one of my servers for blacklist
harvesting. The idea being that anything that comes directly to them
without going to us first is spam.

Thoughts?