You have to fill in your own email address and that is listed as the 'from' information.

from    Nanne xxx <nannexxx@hotmail.com>
to    zwaerdenmaecker@gmail.com,
date    Feb 10, 2008 8:14 PM
subject    US Election - not another Bush
mailed-by    bounces.avaaz.org

GoPetition lists itself as sender, and fills in the reply-to field. Rather less ideal.

from    GoPetition <info@gopetition.com>
reply-to    Nanne xxx <nannexxx@hotmail.com>,
to    zwaerdenmaecker@gmail.com,
date    Feb 10, 2008 11:06 PM
subject    Something interesting from GoPetition