[exim] Sender header gets attached to outgoing email

Hi all,

When i send email using php's mail() function, a "Sender" header gets
attached. When i receive the mail, it looks like:

From: myaddress@???
To: toaddress@???
Sender: www@???

I think this is some anti-spam measure where the domain has to be
"trusted". I tried using the -f parameter but that has no effect. Anyone
can pinpoint me to the right configuration option to remove this Sender
header?

I'm using: Exim 4.5.2 on FreeBSD 6.0 and PHP 5.0.5.

Thanks!

Veronica