[Exim] Mixed Headers Rewrite..

hi all,

        Thanks for the help in the mailbomb problem, now that I have
subscribe to exim user list, I would like to ask if anybody knows
how to rewrite headers for the follwing case (as I could'nt find anyway
in the spec.txt)

>if envelope To field = "foobar@???"

>then rewrite the "To:" = "foobar@???"

this is needed for some strange filtering setup on the local setup, I
don't want to modify the envelope but want to modify only To: depending
on enevolpe.

TIA

ewiz