Message headers describe the route the e-mail message has traveled from the originator to the recipient as well as metadata about the e-mail message.
Here is an example:
Return-Path: <ggeebv@cible-emploi.qc.ca>
Received: from localhost (localhost [127.0.0.1])
by localhost (Postfix) with ESMTP id D43B864620
for <maria_callas@localhost>; Sat, 20 Mar 2004 13:10:44 -0500 (EST)
Received: from smtp.3dresearch.com ([127.0.0.1])
by localhost (dorabella [127.0.0.1]) (amavisd-new, port 10024)
with SMTP id 05719-03 for <maria_callas@localhost>;
Sat, 20 Mar 2004 13:10:44 -0500 (EST)
Received: from stahl-groeditz.de (morris-206-45-168-47.mts.net [206.45.168.47])
by smtp.3dresearch.com (Postfix) with SMTP id 3517F8AE06
for <maria_callas@3dresearch.com>; Sat, 20 Mar 2004 13:10:40 -0500 (EST)
Message-ID: <0fb701c40eab$e06dd242$edd447ee@stahl-groeditz.de>
From: "Gertrude Gee" <ggeebv@cible-emploi.qc.ca>
To: maria_callas@operadiva.com
Subject: (1)Guaranteed to boost size(61)
Date: Sat, 20 Mar 2004 18:45:53 +0000
X-Virus-Scanned: by amavisd-new-20030616-p5 (Debian) at 3dresearch.com
Content-Type: text/html; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
Return-Path: This header is intended to contain definitive information about the address and route back to the message's originator. However, the value of the Return-Path field can easily be forged, therefore, the message sender cannot be reliably determined based on the value of the Return-Path header.
Received: A Received header is added by each mail transport agent (MTA) that relays the message. This header shows when and from where did the MTA receive the message as well as a unique ID. The MTA may also identify itself in this header and indicate the protocol used for receiving the message as well as which account was the message received for.
There are three Received headers in the above example. The third Received header from the top was inserted first and subsequent Received headers were inserted above the preceding Received header. This sequence of Received headers records the "chain of custody" of the message. Received headers can also be forged. However, the IP address is difficult to forge, therefore, Received headers are useful to trace messages' origins.
Message-ID: This header contains a unique identifier. The uniqueness of the message identifier is guaranteed by the host which generates it.
From: The sender's name and email address. It is trivial to forge the From header.
To: The recipient's name and email address.
Date: The date when the message was sent.
Subject: The subject line.
X-Virus-Scanned: Headers beginning with X- are user-defined fields. This header indicates that the message has been scanned for viruses.
Content-Type: The purpose of the Content-Type header is to describe the data contained in the message body so the receiving user agent can pick an appropriate agent or mechanism to present the data to the user, or otherwise deal with the data in an appropriate manner. The value in this field is called a media type (RFC 2045).
Content-Transfer-Encoding: The value of this header describes what mechanism is used to encode the original message in order to faithully reproduce it for the recipient. Encoding of messages often necessary because of restrictive protocols, such as SMTP (RFC 821).
Last modified 2005-09-19 06:09:04.
