How to set the envelope sender address with Pear mail_queue?

Hello here,
I am trying to find out how to set the envelope sender address with the Pear mail_queue system:

http://pear.php.net/package/Mail_Queue/

With the regular PHP mail command you can define that with the -f option:

mail('test@example.org', 'Subject', 'Body', 'From: user@example.org', '-f user@example.org');

But i have no idea how to define that with Pear mail_queue.

Any ideas are very welcome!

Thank you in advance.

Best,
Fab.