[Exim] Simple anonymous remailer - need help

Forgive me if this is a stupid question, but I rarely need to set up
anything more complicated than relay controls and local domains and this
one's a bit beyond my experience.

I'm trying to set up what's essentially a double-blind remailer. It just
does two things to incoming messages:

1. Replace the fake local recipient address with the corresponding real
remote recipient address.
2. Replace the real sender address with the corresponding fake local
address.