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Email headers are the lines at the top of an email message that are used by servers on the Internet as they deliver the message.

Your email program normally shows you only the standard To:, From:, Subject:, and Date: headers, but there are more.

The most important header when you want to complain about spam is Received:, which tells you the route the message took when it was sent to you.

You can instruct most email programs to display the full headers of any message that you receive. While viewing the message,

In Eudora: Click the Blah Blah Blah button.
In Netscape: Select: View->Headers->All
In Outlook: Select: View->Options
In Pine: Type H. (Requires the enable-full-header-cmd feature.)
In WebMail: Click View Full Headers.

The domain names and IP addresses in Received: headers are those of the actual machine that performed the service. These headers can be faked, but it’s harder to do than faking From: addresses.

Received: Headers
The Received: headers of any email message will tell you where the message originated and what route it took to get to you.

That’s what you need to know to complain about spam.