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2. Composing

Creating a MIME message is boring and non-trivial. Therefore, a library called mml has been defined that parses a language called MML (MIME Meta Language) and generates MIME messages.

The main interface function is mml-generate-mime. It will examine the contents of the current (narrowed-to) buffer and return a string containing the MIME message.

2.1 Simple MML Example  An example MML document.
2.2 MML Definition  All valid MML elements.
2.3 Advanced MML Example  Another example MML document.
2.4 Encoding Customization  Variables that affect encoding.
2.5 Charset Translation  How charsets are mapped from MULE to MIME.
2.6 Conversion  Going from MIME to MML and vice versa.
2.7 Flowed text  Soft and hard newlines.

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2.1 Simple MML Example

Here's a simple `multipart/alternative':

<#multipart type=alternative>
This is a plain text part.
<#part type=text/enriched>
<center>This is a centered enriched part</center>

After running this through mml-generate-mime, we get this:

Content-Type: multipart/alternative; boundary="=-=-="


This is a plain text part.

Content-Type: text/enriched

<center>This is a centered enriched part</center>


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2.2 MML Definition

The MML language is very simple. It looks a bit like an SGML application, but it's not.

The main concept of MML is the part. Each part can be of a different type or use a different charset. The way to delineate a part is with a `<#part ...>' tag. Multipart parts can be introduced with the `<#multipart ...>' tag. Parts are ended by the `<#/part>' or `<#/multipart>' tags. Parts started with the `<#part ...>' tags are also closed by the next open tag.

There's also the `<#external ...>' tag. These introduce `external/message-body' parts.

Each tag can contain zero or more parameters on the form `parameter=value'. The values may be enclosed in quotation marks, but that's not necessary unless the value contains white space. So `filename=/home/user/#hello$^yes' is perfectly valid.

The following parameters have meaning in MML; parameters that have no meaning are ignored. The MML parameter names are the same as the MIME parameter names; the things in the parentheses say which header it will be used in.

The MIME type of the part (Content-Type).

Use the contents of the file in the body of the part (Content-Disposition).

The contents of the body of the part are to be encoded in the character set specified (Content-Type). See section 2.5 Charset Translation.

Might be used to suggest a file name if the part is to be saved to a file (Content-Type).

Valid values are `inline' and `attachment' (Content-Disposition).

Valid values are `7bit', `8bit', `quoted-printable' and `base64' (Content-Transfer-Encoding). See section 2.5 Charset Translation.

A description of the part (Content-Description).

RFC822 date when the part was created (Content-Disposition).

RFC822 date when the part was modified (Content-Disposition).

RFC822 date when the part was read (Content-Disposition).

Who to encrypt/sign the part to. This field is used to override any auto-detection based on the To/CC headers.

Identity used to sign the part. This field is used to override the default key used.

The size (in octets) of the part (Content-Disposition).

What technology to sign this MML part with (smime, pgp or pgpmime)

What technology to encrypt this MML part with (smime, pgp or pgpmime)

Parameters for `text/plain':

Formatting parameter for the text, valid values include `fixed' (the default) and `flowed'. Normally you do not specify this manually, since it requires the textual body to be formatted in a special way described in RFC 2646. See section 2.7 Flowed text.

Parameters for `application/octet-stream':

Type of the part; informal--meant for human readers (Content-Type).

Parameters for `message/external-body':

A word indicating the supported access mechanism by which the file may be obtained. Values include `ftp', `anon-ftp', `tftp', `localfile', and `mailserver'. (Content-Type.)

The RFC822 date after which the file may no longer be fetched. (Content-Type.)

The size (in octets) of the file. (Content-Type.)

Valid values are `read' and `read-write' (Content-Type).

Parameters for `sign=smime':

File containing key and certificate for signer.

Parameters for `encrypt=smime':

File containing certificate for recipient.

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2.3 Advanced MML Example

Here's a complex multipart message. It's a `multipart/mixed' that contains many parts, one of which is a `multipart/alternative'.

<#multipart type=mixed>
<#part type=image/jpeg filename=~/rms.jpg disposition=inline>
<#multipart type=alternative>
This is a plain text part.
<#part type=text/enriched name=enriched.txt>
<center>This is a centered enriched part</center>
This is a new plain text part.
<#part disposition=attachment>
This plain text part is an attachment.

And this is the resulting MIME message:

Content-Type: multipart/mixed; boundary="=-=-="


Content-Type: image/jpeg;
Content-Disposition: inline;
Content-Transfer-Encoding: base64


Content-Type: multipart/alternative; boundary="==-=-="


This is a plain text part.

Content-Type: text/enriched;

<center>This is a centered enriched part</center>



This is a new plain text part.

Content-Disposition: attachment

This plain text part is an attachment.


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2.4 Encoding Customization

Mapping from MIME charset to encoding to use. This variable is usually used except, e.g., when other requirements force a specific encoding (digitally signed messages require 7bit encodings). The default is

((iso-2022-jp . 7bit)
 (iso-2022-jp-2 . 7bit)
 (utf-16 . base64)
 (utf-16be . base64)
 (utf-16le . base64))

As an example, if you do not want to have ISO-8859-1 characters quoted-printable encoded, you may add (iso-8859-1 . 8bit) to this variable. You can override this setting on a per-message basis by using the encoding MML tag (see section 2.2 MML Definition).

Prioritize coding systems to use for outgoing messages. The default is nil, which means to use the defaults in Emacs, but is (iso-8859-1 iso-2022-jp iso-2022-jp-2 shift_jis utf-8) when running Emacs in the Japanese language environment. It is a list of coding system symbols (aliases of coding systems are also allowed, use M-x describe-coding-system to make sure you are specifying correct coding system names). For example, if you have configured Emacs to prefer UTF-8, but wish that outgoing messages should be sent in ISO-8859-1 if possible, you can set this variable to (iso-8859-1). You can override this setting on a per-message basis by using the charset MML tag (see section 2.2 MML Definition).

Mapping from MIME types to encoding to use. This variable is usually used except, e.g., when other requirements force a safer encoding (digitally signed messages require 7bit encoding). Besides the normal MIME encodings, qp-or-base64 may be used to indicate that for each case the most efficient of quoted-printable and base64 should be used.

qp-or-base64 has another effect. It will fold long lines so that MIME parts may not be broken by MTA. So do quoted-printable and base64.

Note that it affects body encoding only when a part is a raw forwarded message (which will be made by gnus-summary-mail-forward with the arg 2 for example) or is neither the `text/*' type nor the `message/*' type. Even though in those cases, you can override this setting on a per-message basis by using the encoding MML tag (see section 2.2 MML Definition).

When this is non-nil, it means that textual parts are encoded as quoted-printable if they contain lines longer than 76 characters or starting with "From " in the body. Non-7bit encodings (8bit, binary) are generally disallowed. This reduce the probability that a non-8bit clean MTA or MDA changes the message. This should never be set directly, but bound by other functions when necessary (e.g., when encoding messages that are to be digitally signed).

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2.5 Charset Translation

During translation from MML to MIME, for each MIME part which has been composed inside Emacs, an appropriate charset has to be chosen.

If you are running a non-MULE Emacs, this process is simple: If the part contains any non-ASCII (8-bit) characters, the MIME charset given by mail-parse-charset (a symbol) is used. (Never set this variable directly, though. If you want to change the default charset, please consult the documentation of the package which you use to process MIME messages. See section `Various Message Variables' in Message Manual, for example.) If there are only ASCII characters, the MIME charset US-ASCII is used, of course.

Things are slightly more complicated when running Emacs with MULE support. In this case, a list of the MULE charsets used in the part is obtained, and the MULE charsets are translated to MIME charsets by consulting the table provided by Emacs itself or the variable mm-mime-mule-charset-alist for XEmacs. If this results in a single MIME charset, this is used to encode the part. But if the resulting list of MIME charsets contains more than one element, two things can happen: If it is possible to encode the part via UTF-8, this charset is used. (For this, Emacs must support the utf-8 coding system, and the part must consist entirely of characters which have Unicode counterparts.) If UTF-8 is not available for some reason, the part is split into several ones, so that each one can be encoded with a single MIME charset. The part can only be split at line boundaries, though--if more than one MIME charset is required to encode a single line, it is not possible to encode the part.

When running Emacs with MULE support, the preferences for which coding system to use is inherited from Emacs itself. This means that if Emacs is set up to prefer UTF-8, it will be used when encoding messages. You can modify this by altering the mm-coding-system-priorities variable though (see section 2.4 Encoding Customization).

The charset to be used can be overridden by setting the charset MML tag (see section 2.2 MML Definition) when composing the message.

The encoding of characters (quoted-printable, 8bit etc) is orthogonal to the discussion here, and is controlled by the variables mm-body-charset-encoding-alist and mm-content-transfer-encoding-defaults (see section 2.4 Encoding Customization).

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2.6 Conversion

A (multipart) MIME message can be converted to MML with the mime-to-mml function. It works on the message in the current buffer, and substitutes MML markup for MIME boundaries. Non-textual parts do not have their contents in the buffer, but instead have the contents in separate buffers that are referred to from the MML tags.

An MML message can be converted back to MIME by the mml-to-mime function.

These functions are in certain senses "lossy"---you will not get back an identical message if you run mime-to-mml and then mml-to-mime. Not only will trivial things like the order of the headers differ, but the contents of the headers may also be different. For instance, the original message may use base64 encoding on text, while mml-to-mime may decide to use quoted-printable encoding, and so on.

In essence, however, these two functions should be the inverse of each other. The resulting contents of the message should remain equivalent, if not identical.

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2.7 Flowed text

The Emacs MIME library will respect the use-hard-newlines variable (see section `Hard and Soft Newlines' in Emacs Manual) when encoding a message, and the "format=flowed" Content-Type parameter when decoding a message.

On encoding text, regardless of use-hard-newlines, lines terminated by soft newline characters are filled together and wrapped after the column decided by fill-flowed-encode-column. Quotation marks (matching `^>* ?') are respected. The variable controls how the text will look in a client that does not support flowed text, the default is to wrap after 66 characters. If hard newline characters are not present in the buffer, no flow encoding occurs.

On decoding flowed text, lines with soft newline characters are filled together and wrapped after the column decided by fill-flowed-display-column. The default is to wrap after fill-column.

If non-nil a format=flowed article will be displayed flowed.

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