[exim] Modifying Return-Path header of outgoing messages

Hello everybody.

I'm trying to modify the return-path header of outgoing messages.
Thus I would like to put in this field the value of the "from:" header
specified by my users.
I managed to do it by a rewriting rule but it brings some awful problems.
Indeed, my config file performs some SQL queries (in the ACL section)
that use the variable $sender_address_local_part.
According to the fact that this variable is modified by my rewriting
rule, it results in a failure of the process.

How can I do to change the return_path without altering my SQL queries
in the ACL section?

Best regards.

Johan MEREAUX