Do the Properties, Details tab on the mail and click the Message Source button. Armed with this, use
http://spamcop.com to distinguish the ISP by waiting for the return mail from spamcop and use the "Finish Reporting" link. When you get to the reporting page you scroll down to see where the E-mail originated from.
Does this sound too complicated?
If you want, click on my profile and you can send me an E-mail with the full source code found as described above and I will send it through spamcop for you and give you a reply on the results
Cheers!