[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.

(Sorry about that, managed to hit the 'send' button somehow.)

I can do half of this with a simple alias file. How do I do the sender
rewriting?

Thanks!

Scott