Linux Network Administratoramp;#039;s Guide (3rd Edition) [Electronic resources] نسخه متنی

11.1. What Is a Mail Message?

A mail message generally consists of a message body, which is the
text of the message, and special administrative data specifying
recipients, transport medium, etc., similar to what you see when you
look at a physical letter's envelope.

This administrative
data falls into two categories. In the first category is any data
that is specific to the transport medium, such as the address of
sender and recipient. It is therefore called the
envelope. It may be transformed by the transport
software as the message is passed along.

The second variety is any data
necessary for handling the mail message, which is not particular to
any transport mechanism, such as the message's
subject line, a list of all recipients, and the date the message was
sent. In many networks, it has become standard to prepend this data
to the mail message, forming the so-called mail
header. It is offset from the
mail body by an empty
line.[2] Most mail transport
software in the Unix world use a header format outlined in RFC 822.
Its original purpose was to specify a standard for use on the
ARPANET, but since it was designed to be independent from any
environment, it has been easily adapted to other networks, including
many UUCP-based networks.

[2] It is customary to append a
signature or .sig to a mail
message, usually containing information on the author. It is offset
from the mail message by a line containing
"-- ", followed
by a space. Netiquette dictates, "Keep it
short."

RFC 822 is only the lowest common
denominator, however. More recent standards have been conceived to
cope with growing needs such as data encryption, international
character set support, and Multipurpose Internet Mail Extensions
(MIME), described in RFC 1341 and other RFCs.

In all these standards, the header consists
of several lines separated by an end-of-line sequence. A line is made
up of a field name, beginning in column one, and the field itself,
offset by a colon and whitespace. The format and semantics of each
field vary depending on the field name. A header field can be
continued across a newline if the next line begins with a whitespace
character such as tab. Fields can appear in any order.

A typical mail header may look like this:

Return-Path: <root@oreilly.com>
X-Original-To: spam@xtivix.com
Delivered-To: spam@ xtivix.com
Received: from smtp2.oreilly.com (smtp2.oreilly.com [209.58.173.10])
by www.berkeleywireless.net (Postfix) with ESMTP id B05C520DF0A
for <spam@ xtivix.com>; Wed, 16 Jul 2003 06:08:44 -0700 (PDT)
Received: (from root@localhost)
by smtp2.oreilly.com (8.11.2/8.11.2) id h6GD5f920140;
Wed, 16 Jul 2003 09:05:41 -0400 (EDT)
Date: Wed, 16 Jul 2003 09:05:41 -0400 (EDT)
Message-Id: <200307161305.h6GD5f920140@smtp2.oreilly.com>
From: Andy Oram <root@oreilly.com>
To: spam@ xtivix.com
Subject: Article on IPv6

Usually, all necessary header fields are
generated by the mail reader you use, such as
elm, Outlook,
Evolution, or pine.
However, some are optional and may be added by the user.
elm, for example, allows you to edit part of the
message header. Others are added by the mail transport software. If
you look into a local mailbox file, you may see
each mail message preceded by a
"From" line (note: no colon). This
is not an RFC 822 header; it has been inserted by your mail software
as a convenience to programs reading the mailbox. To avoid potential
trouble with lines in the message body that also begin with
"From," it has become standard
procedure to escape any such occurrence in the body of a mail message
by preceding it with a > character.

This list is a collection of common header fields and their meanings:

From:

This contains the sender's email address and
possibly the "real name." Many
different formats are used here, as almost every mailer wants to do
this a different way.

To:

This is a list of recipient email addresses. Multiple recipient
addresses are separated by a comma.

Cc:

This is a list of email addresses that will receive
"carbon copies" of the message.
Multiple recipient addresses are separated by a comma.

Bcc:

This is a list of hidden email addresses that will receive
"carbon copies" of the message. The
key difference between a "Cc:" and
a "Bcc:" is that the addresses
listed in a "Bcc:" will not appear
in the header of the mail messages delivered to any recipient.
It's a way of alerting recipients that
you've sent copies of the message to other people
without telling the others. Multiple recipient addresses are
separated by a comma.

Subject:

Describes the content of the mail in a few words.

Date:

Supplies the date and time the mail was sent.

Reply-To:

Specifies the address that the sender wants the
recipient's reply directed to. This may be useful if
you have several accounts, but want to receive the bulk of mail only
on the one you use most frequently. This field is optional.

Organization:

The organization that owns the machine from which the mail
originates. If your machine is owned by you privately, either leave
this out, or insert "private" or
some complete nonsense. This field is not described by any RFC and is
completely optional. Some mail programs support it directly, many
don't.

Message-ID:

A string generated by the mail transport on the originating system.
It uniquely identifies this message.

Received:

Every site that processes your mail (including the machines of sender
and recipient) inserts such a field into the header, giving its site
name, a message ID, time and date it received the message, which site
it is from, and which transport software was used. These lines allow
you to trace which route the message took, and you can complain to
the person responsible if something went wrong.

X- anything:

No mail-related programs should complain about any header that starts
with X-. It is used to implement additional
features that have not yet made it into an RFC, or never will. For
example, there was once a very large Linux mailing list server that
allowed you to specify which channel you wanted the mail to go to by
adding the string X-Mn-Key: followed by the
channel name.