[exim] Processing messages with an external program (script)

Greetings,

I need to pass outgoing (outgoing only) message to an external
program, to make certain header transformations (namely, canonize
certain To/From addresses) prior to their being actually sent. Could
someone offer me the hints on how to do that?

The program/script expects message on stdin and sends modified message
to stdout for further processing/delivery.

Thanks!