Headers are lines at the beginning of an email message that are used by servers to trace that message's delivery path. Knowing how to interpret these headers is a useful skill. You can use full headers to determine exactly where a message came from, as well as whether or not the person in the From: field actually sent the message.

Enabling full header display
Under normal circumstances, your email client will only display the standard To:, From:, Subject:, and Date: headers. These headers are all easily forged by spammers or virus-infected computers. They are rarely helpful in determining a message's true origins. 

In order to reveal the full headers, you will have to enable their display within your email client:

• Mozilla - Go to the View menu. Choose Headers --> All.
• Outlook - Go to the View menu. Choose Options. The headers appear in the Internet headers box.
• Pine - Hit the "H" key to toggle full header displays.
• Thunderbird - Go to the View menu. Choose Headers --> All.

If your client is not listed above, consult the built-in help or online documentation for your email client.

Interpreting full headers
The Received: headers of any email message will tell you where a message originated and which servers it passed through before it arrived in your inbox. Received: headers should be read in reverse order (from the bottom line to the top line).

For a very comprehensive and easy-to-understand explanation of how to read a full set of email headers, we recommend the Reading Email Headers page on the Stopspam.org website.