[exim] allow return-path header, thought local SMTP

Hi,

I'm trying to send email from a script (lib PHP Zend_Mail, from
ZendFramework) to an Exim 4.69 server.
I send thought the localhost:25 SMTP socket (not thought
/usr/sbin/sendmail), and my email contains a "Return-Path: xxxx" header,
but Exim replace it by the content of the "From: xxxx" header.

I haven't got any "return_path_add" in my "remote_smtp" transport.
And I tried to set "return_path_remove = false", without success.

How can I fix that ?

Thanks,
Olivier