Step 1:
If you're using OE right click on the email.
Goto properties.
Goto details.
Under received you should have most info you'll need but the problem is you don't know if the supplied IP is from the user, his ISP, his ISP gateway, email server, etc.
Step 2:
Once you track back to the email server.
You can email the admin all the information listed in Step 1.
If you get a reply from the admin they would give you the IP address of the email account that sent you that email.
Step 3:
Email the ISP admin details from Step 1 and Step 2.
They will check the IP address is coming from which gateway/cvt then they will know which account login under that IP address, then they can check in their customer database.
Of course its tedious, need co-operation from admins and a very good reason why they should help you.