[exim] rewrite envelope from when forward

Hi

When I forward an address to an @bluewin.ch address, they bring back an
error like this:

SMTP error from remote mail server after end of data:
554 5.2.0 sc971: SPF hard fail

I understood this is because the envelope from domain is not SPF proofed
to send them a message over my server.

I thought I would solve this by change the envelope from address to the
address of the forwarding user. SRS let me change addresses. But how can
I write the envelope to address to the envelope from address. I tried to
do that while processing out (transport) but then it says, it's not
possible to change the envelope address at this stage.


Here is an example: Mike is sending a mail to a user of mine (Lisa), who
is forwarding her address to an external hoster (bluewin). Because Mike
is not allowed to send mails from my server to bluewin denies the mail.

mike@??? --> lisa@???
lisa forwards her mails to lisa@??? on my server
the mail is blocked by bluewin


I'm not professional in exim and mailing, but I think this should be
realised for everyone...?
Thanks for any help!
Johannes