[exim-dev] headers_rewrite

Hello, good morning everyone.
We are currently using exim on some cPanel servers we have, we are
implementing a central relay to improve email management and sending speed.
This relay only allows previously registered, authorized and authenticated
domains (SPF, DKIM) to send new messages.

My problem is with forwarders, when a person creates a forwarder, eg:

1- xxxxxxxxx@??? - Original Sender
2- xxx@??? - Original Recipient
3- xxxxx@??? - Forward Recipient

We have a problem relaying, because domain 1 is not on the list of allowed
domains in the relay.
After much research I was able to find something referring to
'headers_rewrite',
but as I don't have a thorough knowledge of exim, I can't change the from
field, so that the sender is in fact number 2, not number 1.

We have also tried SRS, but it did not work on re-transmission.

Could someone help me solve this?