Email headers, example of email header, information inside headers

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What is an email header

The email header is the information that travels with every email, containing details about the sender, route and receiver. It is like a flight ticket: it can tell you who booked it (who sent the email), the departure information (when the email was sent), the route (from where it was sent and how did it arrive to you) and arrival details (who is the receiver and when it was received). As when you would book a flight ticket with a false identity, the same goes for emails: the sender can partially fake these details, pretending that the email was sent from a different account (common practice for spammers or viruses).

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: <example_from@abc.com>
X-SpamCatcher-Score: 1 [X]
Received: from [136.167.40.119] (HELO abc.com)
    by fe3.abc.com (CommuniGate Pro SMTP 4.1.8)
    with ESMTP-TLS id 61258719 for example_to@mail.abc.com;
Message-ID: <4129F3CA.2020509@abc.com>
Date: Wed, 21 Jan 2009 12:52:00 -0500 (EST)
From: Taylor Evans <example_from@abc.com>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.0.1)
X-Accept-Language: en-us, en
MIME-Version: 1.0
To: Jon Smith <example_to@mail.abc.com>
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.