Below are various lessons written by Bill Mattocks on how to identify the source of spam:
If you are lazy then you can also use the Spamcop service, where you can paste the entire email and it will automatically find the spammer's ISP and web hosting company and send out complaints.
Now that you've learned how to track the source of the spam, here's my lesson on how to respond.
After you have read the above, here are some spam tracking tools. I personally use Sam Spade, Spam hunter on a frequent basis.
Here are some other links on reading Email headers