[exim] Special Relaying needed

Hi,
I want a machine with exim to act as smarthost for another machine in
the same network. The first machine on the other hand also delivering by
an master smarthost, and only this machine is allowed for relaying over
the master smarthost.

In principle this setup is running fine ;) but the mails from the
machine, which is not allowed for direct usage of the master smarthost,
aren't delivered (of course, its ip and hostname appearing in the helo).

Is there a possibility to rewrite / fake the whole helo in the smtp
transport?

Regards
Ron