To determine the source of an email, you will need to examine the email headers. Email headers look something like this:

Learning how to read and interpret the mail headers is the key to determining the source of the email. If you do not know how to access the email headers in your mail program, this FAQ from the SpamCop.net web site explains how to access the email headers in many email clients. The tutorials below will help you learn how to interpret the email headers.

Tutorials and Resources on Interpreting Email Headers:
EmailAbuse.org: Identify the Sender
PC Magazine (PCMAG.COM): Heading Off Spam
About.com: What Email Headers Can Tell You About the Origin of Spam