[Exim] replacing text in the message body

Hello,

I have got the following problem.
When sending messages as replay there are somteimes
internal email adresses (local adresses) which can not be uses as a
replay adress. but my recipients use them for replies.
how can I replaxe those local adresses before sending a mail?
e.g. there is hans@localhost in the body of the mail and I want it to be
hans@??? before sending the email outside my local net.

any ideas ?

thanks in advance

hans