installed returnpath and activated module, but returnpath in mails sent still not ok

Hi,
I just installed and activated the returnpath module on my drupal 5.1 site.
I sent some test emails through contact form and simplenews, the returnpath in the mailheader is still as before, some weird internal mailadres of my webhost instead of the "from" address.
Did I forget some setting?

Regards,
Ben