148 lines
7.1 KiB
HTML
148 lines
7.1 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<!-- updated Sun Jul 14 20:05:00 MET DST 1996 by:
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Tony den Haan (tony@iaehv.nl) ftp://ftp.IAEhv.nl/pub/users/tony-->
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<HEAD>
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<TITLE>What is a Mail Message?</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node188.html">How is Mail Delivered?</A>
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<B>Up:</B> <A HREF="node186.html">Electronic Mail</A>
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<B> Previous:</B> <A HREF="node186.html">Electronic Mail</A>
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<BR> <P>
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<H1><A NAME="SECTION0015100000">What is a Mail Message?</A></H1>
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<P>
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<A NAME="mailmessageformat"></A>
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A Mail message generally consists of a message body, which is the text
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the sender wrote, and special data specifying recipients, transport
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medium, etc., very much like what you see when you look at a letter's
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envelope.
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<P>
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This administrative data falls into two categories; in the first
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category is any data that is specific to the transport medium, like
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the address of sender and recipient. It is therefore called <em>the
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envelope</em>. It may be transformed by the transport software as the
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message is passed along.
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<P>
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The second variety is any data necessary for handling the mail message,
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which is not particular to any transport mechanism, such as the
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message's subject line, a list of all recipients, and the date the
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message was sent. In many networks, it has become standard to prepend
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this data to the mail message, forming the so-called <em>mail header</em>.
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It is offset from the <em>mail body</em> by an empty line.<A HREF="footnode.html#7158"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A>
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<P>
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Most mail transport software in the world uses a header format
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outlined in a RFC-822. Its original purpose was to specify a standard
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for use on the ARPANET, but since it was designed to be independent from
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any environment, it has been easily adapted to other networks, including
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many UUCP-based networks.
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<P>
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RFC-822 however is only the greatest common denominator. More recent
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standards have been conceived to cope with growing needs as, for
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example, data encryption, international character set support, and
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multi-media mail extensions (MIME).
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<P>
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In all these standards, the header consists of several lines, separated
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by newline characters. A line is made up of a field name, beginning in
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column one, and the field itself, offset by a colon and white space. The
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format and semantics of each field vary depending on the field name. A
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header field may be continued across a newline, if the next line begins
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with a TAB. Fields can appear in any order.
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<P>
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A typical mail header may look like this:
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<PRE>
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From brewhq.swb.de!ora.com!andyo Wed Apr 13 00:17:03 1994
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Return-Path: <brewhq.swb.de!ora.com!andyo>
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Received: from brewhq.swb.de by monad.swb.de with uucp
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(Smail3.1.28.1 #6) id m0pqqlT-00023aB; Wed, 13 Apr 94 00:17
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Received: from ora.com (ruby.ora.com) by brewhq.swb.de with smtp
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(Smail3.1.28.1 #28.6) id <m0pqoQr-0008qhC>; Tue, 12 Apr 94 2
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Received: by ruby.ora.com (8.6.8/8.6.4) id RAA26438; Tue, 12 Apr 94
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Date: Tue, 12 Apr 1994 15:56:49 -0400
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Message-Id: <199404121956.PAA07787@ruby>
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From: andyo@ora.com (Andy Oram)
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To: okir@monad.swb.de
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Subject: Re: Your RPC section
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</PRE>
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Usually, all necessary header fields are generated by the mailer
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interface you use, like elm, pine, mush, or
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mailx. Some however are optional, and may be added by the user.
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elm, for example, allows you to edit part of the message header.
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Others are added by the mail transport software. A list of common
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header fields and their meaning are given below:
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<PRE>
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From: This contains the sender's email address, and possibly the
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``real name''. A complete zoo of formats is used here.
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To: This is the recipient's email address.
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Subject: Describes the content of the mail in a few words. At least
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that's what it should do.
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Date: The date the mail was sent.
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Reply-To: Specifies the address the sender wants the recipient's reply
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directed to. This may be useful if you have several accounts,
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but want to receive the bulk of mail only on the one you use
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most frequently. This field is optional.
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Organization: The organization that owns the machine from which the mail
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originates. If your machine is owned by you privately, either
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leave this out, or insert ``private'' or some complete non-
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sense. This field is optional.
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Message-ID: A string generated by mail transport on the originating sys-
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tem. It is unique to this message.
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Received: Every site that processes your mail (including the machines of
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sender and recipient) inserts such a field into the header,
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giving its site name, a message id, time and date it received
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the message, which site it is from, and which transport soft-
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ware was used. This is so that you can trace which route the
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message took, and can complain to the person responsible if
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something went wrong.
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X-anything: No mail-related programs should complain about any header
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which starts with X-. It is used to implement additional fea-
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tures that have not yet made it into an RFC, or never will.
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This is used by the Linux Activists mailing list, for example,
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where the channel is selected by the X-Mn-Key: header field.
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</PRE><P>
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The one exception to this structure is the very first line. It starts
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with the keyword From which is followed by a blank instead of a
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colon. To distinguish it from the ordinary From: field, it is
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frequently referred to as From_. It contains the route the
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message has taken in UUCP bang-path style (explained below), time and
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date when it was received by the last machine having processed it, and
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an optional part specifying which host it was received from. Since this
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field is regenerated by every system that processes the message, it is
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sometimes subsumed under the envelope data.
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<P>
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The From_ field is there for backward compatibility with some
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older mailers, but is not used very much anymore, except by
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mail user interfaces that rely on it to mark the beginning of a
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message in the user's mailbox. To avoid potential trouble with lines
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in the message body that begin with ``From '', too, it has become
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standard procedure to escape any such occurrence by preceding it with
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``>''.
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<P>
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<HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node188.html">How is Mail Delivered?</A>
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<B>Up:</B> <A HREF="node186.html">Electronic Mail</A>
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<B> Previous:</B> <A HREF="node186.html">Electronic Mail</A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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</BODY>
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</HTML>
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