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<META NAME="Author" CONTENT="Jerry Peek">
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<TITLE>MH & nmh: Overview of MIME Messages</TITLE>
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<H1>Overview of MIME Messages</H1>
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[<A HREF="ch-itm.htm">previous</A>]
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[<A HREF="mulmes.htm">next</A>]
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[<A HREF="tocs/jump.htm">table of contents</A>] [<A HREF="indexes/map.htm">index</A>]
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This section starts by explaining the overall purposes of MIME.
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Next it shows a simple MIME message and introduces
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the basic parts of a MIME message.
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<H2><A NAME="PuofMI">Purposes of MIME</A></H2>
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Make "tree-structured" message bodies that have many levels of parts
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This is like the idea of a UNIX filesystem, an office file cabinet,
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This lets you send many different things (messages, graphics, sounds,
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and so on) in the same message.
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Like non-MIME messages, though, many MIME messages have just one part.
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Handle multiple character sets, even within the same mail message.
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Transport many different types of data in the same message.
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Pass data reliably through standard mail transports.
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The sender's MIME agent converts text and data into 7-bit <I>us-ascii</I>
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with line length limits.
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The recipient's MIME-capable mail program converts the message contents
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into their original formats.
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Let's look at a MIME message as it would arrive in your
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The message in <A HREF="#EncMIMes">the next Example</A>
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was encoded by the sender; this
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is how it is actually stored on disk by the recipient's MTA (mail
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transfer agent), waiting for someone to read it.
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This is also how the message will look to a person who does
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<I>not</I> have a MIME-capable MUA (mail user agent):
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<A NAME="EncMIMes"><EM>Example: Encoded MIME message</EM></A><BR>
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From: Jerry Peek <jpeek@jpeek.com>
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To: carlos@entelfam.cl
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Subject: Un =?iso-8859-1?Q?d=EDa_dif=EDcil?=
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Content-Type: text/plain; charset=ISO-8859-1
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Content-Transfer-Encoding: quoted-printable
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Carlos, estoy en la casa de mi amigo. Pero, =A1qu=E9 d=EDa dif=EDcil!
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Tom=E9 un taxi entre al aeropuerto y el hotel. "=A1Tenga cuidado!",
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me dijo el chofer. "=A1Esta parte de la ciudad es peligrosa!" Fue
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evidente para m=ED. Yo o=EDa que la ciudad tiene partes malas, y esa
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parte pareci=F3 as=ED. Los edificios ten=EDan barrotes sobre sus
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ventanas, hubo gente sospechosa en la calle, y todas las puertas estaban
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cerrada con llave. "=A1Vaya con di=F3s!" =C9l se fue.
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Before we dig into that message, here are some things you should be aware of.
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RFC 1521, the MIME specification that MH supports, only tells how to
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transfer non-ASCII text and graphics in the message <I>body</I>.
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Although RFC 1521 adds fields like <TT>MIME-Version:</TT> to a message
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header, it doesn't tell how to put non-ASCII text in a message header.
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A different specification, RFC 1522,
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is a standard for non-ASCII text in the header.
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RFC 1521 has been replaced by RFC 2045, and RFC 1522 has been replaced
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There are links from the <A HREF="ap-rl.htm">Reference List</A>.
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MH doesn't support RFC 1522 character encoding in message headers
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nmh does support it (actually, RFC 2047) in messages you read, but it
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doesn't yet have automated support for composing messages using those
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characters; you have to type them in by hand as you compose the message.
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To make my message <TT>Subject:</TT> more readable, I could have
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written an ASCII approximation of the non-ASCII characters.
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That is, instead of <TT>Un día difícil</TT>, I'd write
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<TT>Un d'ia dif'icil</TT>.
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Most modern MUAs support RFC 2047, though, so
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let's hope that nmh can have complete support soon.
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<A NAME="index12"></A>
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<H2><A NAME="MIHeFi">MIME Header Fields</A></H2>
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The header field <TT>MIME-Version: 1.0</TT> in
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<A HREF="#EncMIMes">the previous Example</A>
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tells that the message is in MIME format.
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It's a signal to mail programs that the
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message meets the requirements of RFC 1521.
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(Unfortunately, a few mail programs add that header field to messages
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that aren't in MIME format.)
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Here's an overview of the other header fields that a MIME message
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I'll toss in some MIME philosophy along the way.
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We can't cover everything in the MIME spec; it's close to 100 pages long!
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Other sections of this chapter, and sections of later Chapters,
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have more MIME information.
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The Section <A HREF="moabmi.htm">More About MIME</A>
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explains how to find all the gory details.
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<TT>Content-type:</TT> tells you what kind of data is in the message.
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The content can be <I>text</I>, <I>image</I> (still pictures), <I>video</I>,
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Another content type is <I>message</I>, which means that the content is
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structured in the standard RFC 822 format; this can be used for
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The <I>application</I> content type is designed to be sent to an external
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program -- for example, text for a PostScript printer or viewer.
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<A NAME="index4"></A>
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Finally, a message can be <I>multipart</I>, with several separate body parts.
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It's possible for the parts to be of different types (the Section on
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<A HREF="mulmes.htm">Multipart Messages</A>
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explains the MIME syntax for multipart messages).
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For example, a message could start with a <I>text</I> part to describe what's
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in the message, then an <I>audio</I> part with a message from a photographer,
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followed by five of the photographer's pictures (in five <I>image</I> parts).
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It's also possible for each of the <I>multipart</I> body parts to be another
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complete multipart message -- with its own parts.
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That is, MIME messages can be recursive.
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A <TT>Content-type:</TT> must have a subtype.
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The type and subtype names have a slash (<TT>/</TT>) between them.
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For instance, <I>image/gif</I> is a picture in the GIF format; the type is
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<I>image</I> and the subtype is <I>gif</I>.
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The <A HREF="mimerg.htm">MIME Reference Guide</A> lists many of the
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common content types and subtypes.
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<A NAME="index13"></A>
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Finally, a <TT>Content-type:</TT> can have optional parameters at the end,
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starting with a semicolon (<TT>;</TT>).
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For example, the <I>charset=</I> parameter in
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Content-type: text/plain; charset=iso-8859-1
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says that the message body uses the ISO-8859-1 character set.
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(The default <I>charset</I> is <I>us-ascii</I>.)
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The default <TT>Content-type:</TT> is <I>text/plain</I>.
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The <TT>Content-transfer-encoding:</TT> field tells how the message
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data was encoded for transfer.
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Encoding gets the message safely from the person who sent it, across mail
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transfer agents and gateways, into your mailbox.
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The Section <A HREF="#MIMEnc">MIME Encoding</A>
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lists the values that can go in the
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<TT>Content-transfer-encoding:</TT> field.
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The <TT>Content-ID:</TT> and <TT>Content-Description:</TT> header fields
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help to describe what's in the message body.
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These two fields are most useful for the parts of a multipart message.
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The <TT>Content-ID:</TT> is a unique string, similar to the RFC 822 field
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<TT>Message-ID:</TT>, that no other message in the world is likely to have.
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A typical <TT>Content-ID:</TT> value is <TT><1283.780402430.1@ora.com></TT>.
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Each part of a multipart message has its own <TT>Content-ID:</TT>.
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Its main use is identifying
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<A HREF="../mh/remime.htm#ExtParts">external parts</A>
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and <A HREF="../mh/remime.htm#CachCont">cached body parts</A>.
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The <TT>Content-Description:</TT> field describes the content in words, like
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<TT>Report on Zeta Meeting</TT>.
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The RFC 822 <TT>Subject:</TT> field describes the whole message.
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You can add a <TT>Content-Description:</TT> in the message header,
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but it's more useful for describing a part of a multipart message.
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The list of content subtypes changes a lot.
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Quite a few subtypes are for non-UNIX computers.
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This book doesn't cover every content subtype; examples use some
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of the most common subtypes from RFC 1521.
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Your MH setup probably doesn't need to support all subtypes:
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When you send a message, unless you know that the recipient needs
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a particular kind of content, it's a good idea to use the simplest
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(most-widely-available) content type and subtype that you can.
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For instance, to send a non-text data file, you should probably use
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<I>application/octet-stream</I> instead of <I>application/mac-binhex40</I>
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because the BinHex format is generally used for Macintosh files.
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If you get a message with a content that your MH hasn't been set up to
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handle, you may be able to make sense of the message by ignoring the
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<A HREF="../mh/remime.htm#Alttomhn">prevent MIME decoding</A>
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by setting the MH <I>NOMHNPROC</I> environment variable
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or using the nmh <I>-nocheckmime</I> switch.
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Look at the message on your screen.
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If that isn't enough, and you don't think you'll get many messages of
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<A HREF="../mh/stormess.htm#DeStMIMe">Store the decoded message content in a file</A>,
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Take the encoded message apart with a
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text editor (as in the Section
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<A HREF="../mh/edmeshm.htm">Edit Messages with show: mhedit</A>)
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and find a decoder program, or
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Ask the sender to give you the data in a different format.
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In my opinion, it doesn't make sense to constantly reconfigure MH for every
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new content subtype unless you have a lot of spare time. <TT>:-)</TT>
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<A NAME="index5"></A>
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<H2><A NAME="MIMEnc">MIME Encoding</A></H2>
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MIME messages are designed to be readable by all existing
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RFC 822-compatible mail programs.
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(Although, of course, MUAs that don't understand MIME won't be
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able to interpret the MIME-specific parts.)
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The messages may be sent through all kinds of networks and gateways.
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So, MIME encodes messages that have non-ASCII parts.
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The <TT>Content-transfer-encoding:</TT>
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field tells the recipient the way a message
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was encoded -- and how to decode it.
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(Encoding usually isn't required for plain ASCII text.)
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<A HREF="#EncMIMes">the previous Example message</A>,
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from me to a friend in Chile, is in Spanish (more or less <TT>:-)</TT>).
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Spanish uses characters like
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¡ and ñ that aren't part of the ASCII character set.
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My MIME-capable MUA could encode the
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message's non-ASCII characters to pass safely through an
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ASCII-only mail transfer system.
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For instance, the character ¡
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was encoded as the three-character sequence <TT>=A1</TT>.
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When the message gets to Chile, my friend's MIME-capable
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mail reader will translate the message's encoded characters to the correct
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One important feature of MIME's encodings is that they are designed to
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leave as much of the message in plain ASCII text as possible.
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In general, MIME only translates the characters that some email transfer
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So, if the recipient doesn't have a MIME-capable MUA,
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the encoded text in the message will probably still make some sense.
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(It's possible to encode text messages so that people can't read
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them without decoding.
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But, unless you want to hide words or have another reason, one of the
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less-severe encodings will probably do the job.
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In general, MH chooses human-readable encoding for text messages.)
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Of course, when MIME encodes a binary file (like a digitized picture)
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that people can't read in the first place, the encoded data won't be
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any easier for a person to read.
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MIME encoding is designed to get the data safely through almost every
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known mail transfer system and gateway.
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One of the major wins in MIME is that it was designed to work <I>everywhere</I>,
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including "broken" and "brain-damaged" systems.
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Instead of trying to impose a new standard on mail transfer systems, MIME
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works with existing systems -- and adapts to their eccentricities.
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Although you don't need to understand how encoding works to use MIME,
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you should have a general idea of the types of encoding.
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So, if you'd like to skip the technical details in the following
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section, please do skim it and learn the types of encoding.
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There are five encodings:
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<A NAME="index6"></A>
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<B>7bit</B> is the default.
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It means that the message contents are plain ASCII text.
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Lines must be "short" (1000 characters or less) and end with
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CR-LF (carriage return plus linefeed).
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<A NAME="index7"></A>
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<B>quoted-printable</B>
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is used for text that is mostly 7-bit but which has a
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small percentage of 8-bit characters.
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(There's quoted-printable text in
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<A HREF="#EncMIMes">the next Example</A>.)
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For instance, characters with the eighth bit on in the ISO-8859-<I>n</I>
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sets should be encoded as quoted-printable.
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Each 8-bit character is encoded into three 7-bit characters: <TT>=</TT>
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(an equal sign) and the hexadecimal value of the character.
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So the ISO-8859-1 character ñ,
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which has the hex value F1 (that's 11110001 binary),
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would be encoded as <TT>=F1</TT>.
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To keep the message readable on non-MIME readers, characters that don't have
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the eighth bit set generally shouldn't be encoded.
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The <TT>=</TT> character itself must be encoded, though; it's encoded as
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Also, space and tab characters at the ends of lines must be encoded
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(as <TT>=20</TT> and <TT>=09</TT>, respectively); this keeps broken gateways
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If a line ends with <TT>=</TT> followed by CR-LF, those characters are
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ignored; this lets you continue ("wrap") a long line.
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Lines must be no more than 76 characters long, not counting the final
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Longer lines will be broken when the message is encoded and joined
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Quoted-printable text was designed to be (mostly) readable by people
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with non-MIME mail programs.
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<A NAME="index8"></A>
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is used for data and other text that was never meant to be
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read by humans -- or must be preserved verbatim.
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Every 3 octets (24 bits) are encoded into a 4-character sequence.
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The 64-character set was chosen carefully.
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It comes from ASCII characters that aren't munged by known gateways or
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<A NAME="index9"></A>
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is data made of 8-bit characters with "short" lines that end
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This isn't too useful yet because <I>8bit</I> data can't be shipped reliably
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over standard SMTP mail transport.
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The new ESMTP standard (RFC 1651) and <I>8bitMIMEtransport</I>
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extension (RFC 1652) handle 8-bit MIME messages.
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<A NAME="index10"></A>
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is like 8bit, but without CR-LF line boundaries.
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One of MIME's main goals is to make different email
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programs work with each other.
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To make interoperability more likely, the MIME designers tried to avoid
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having lots of different content-types.
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They tried even harder to avoid lots of different encodings.
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MIME content types and subtypes, as well as encodings, are registered with
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the IANA (Internet Assigned Numbers Authority).
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"Experimental" unofficial values start with <I>X-</I>, like <I>X-pbm</I>.
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A few experimental content-types and subtypes are in wide use.
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But in general, so that as many people as possible can read your message,
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try to avoid inventing new content types and subtypes.
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If you get the urge to create, the
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<A HREF="moabmi.htm">
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<I>comp.mail.mime</I> newsgroup and the <I>info-mime</I> mailing list</A>
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are great places to work it out.
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[<A HREF="tocs/jump.htm">Table of Contents</A>] [<A HREF="indexes/map.htm">Index</A>]
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[<A HREF="ch-itm.htm">Previous: Caution About MH Files and Newline Characters</A>]
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[<A HREF="mulmes.htm">Next: Multipart Messages</A>]
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<STRONG>Revised by Jerry Peek.</STRONG>
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<EM>Last change $Date: 1999/10/10 05:14:05 $</EM>
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This file is from the third edition of the book <I>MH & xmh: Email
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for Users & Programmers</I>, ISBN 1-56592-093-7, by Jerry Peek.
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Copyright © 1991, 1992, 1995 by O'Reilly & Associates, Inc.
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This file is freely available; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation. For more information, see
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<A HREF="../copying.htm">the file <I>copying.htm</I></A>.
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Suggestions are welcome:
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<A HREF="http://www.jpeek.com/">Jerry Peek</A>
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<A HREF="mailto:jpeek@jpeek.com"><jpeek@jpeek.com></A>
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