Change the envelope from

Hi,
How can I change the Envelope From ? It is use when the 'to' email is bounce, but, by default, it's the 'from' email. I don't want that. I want the bounce to be delivered to bounce@mydomain.com and not to admin@mydomain.com.
But I do not find the way. I tryied to change the mail header, but it is useless.

http://us2.php.net/manual/en/function.mail.php (additional_parameters (optional))

Thanks a lot