Header

In an e-mail, the body (content text) is always preceded by header lines that identify particular routing information of the message, including the sender, recipient, date and subject. Some headers are mandatory, such as the FROM, TO and DATE headers. Others are optional, but very commonly used, such as SUBJECT and CC. Other headers include the sending time stamps and the receiving time stamps of all mail transfer agents that have received and sent the message. In other words, any time a message is transferred from one user to another (i.e. when it is sent or forwarded), the message is date/time stamped by a mail transfer agent (MTA) - a computer program or software agent that facilitates the transfer of email message from one computer to another. This date/time stamp, like FROM, TO, and SUBJECT, becomes one of the many headers that precede the body of an email.

To really understand what an email header is, you must see one. Here is an example of a full email header*:

Return-Path:
X-SpamCatcher-Score: 1 [X]
Received: from [136.167.40.119] (HELO dc.edu)
by fe3.dc.edu (CommuniGate Pro SMTP 4.1.8)
with ESMTP-TLS id 61258719 for example_to@mail.dc.edu; Mon, 23 Aug 2004 11:40:10 -0400
Message-ID: <4129F3CA.2020509@dc.edu>
Date: Mon, 23 Aug 2005 11:40:36 -0400
From: Taylor Evans
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0
X-Accept-Language: en-us, en
MIME-Version: 1.0
To: Jon Smith
Subject: Business Development Meeting
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
* email headers should always be read from bottom to top.

Fortunately, most of this information is hidden inside the email with only the most relevant or mandatory headers appearing to the user. Those headers that we most often see and recognize are bolded in the above example.