[Exim] [newbie]How to rewrite the "To" header when do forwar…

Hi,

I mantain a local mail address with the file .forward and when i do...
send mail from a@??? to b@???
b@mydomain forwarded to a addresses listed in .forward

the mails forwarded well but the problem is in every delivered mail
the header "To" was b@???, how can i rewrite it to be
the real destination address, not the forwarding address?
thanks

--
Death Marks don't come with the rain.