The raw RFC [2]822 email headers should indicate the route that the email took to get to you and may divulge the ISP used - assuming that it's not simply spam using spoofed headers/IP addresses. Most emai client applications provide some means of looking at the raw message headers which are normally not displayed as a matter of course.
If you do not feel confident about reading email headers yourself you can also use a tool from Visualware www.visualware.com that will trace back the origion of the email.