+@acronym{MIME} messages.
+
+This manual is directed at users who want to modify the behavior of
+the @acronym{MIME} encoding/decoding process or want a more detailed
+picture of how the Emacs @acronym{MIME} library works, and people who want
+to write functions and commands that manipulate @acronym{MIME} elements.
+
+@acronym{MIME} is short for @dfn{Multipurpose Internet Mail Extensions}.
+This standard is documented in a number of RFCs; mainly RFC2045 (Format
+of Internet Message Bodies), RFC2046 (Media Types), RFC2047 (Message
+Header Extensions for Non-@acronym{ASCII} Text), RFC2048 (Registration
+Procedures), RFC2049 (Conformance Criteria and Examples). It is highly
+recommended that anyone who intends writing @acronym{MIME}-compliant software
+read at least RFC2045 and RFC2047.
+
+@menu
+* Decoding and Viewing:: A framework for decoding and viewing.
+* Composing:: @acronym{MML}; a language for describing @acronym{MIME} parts.
+* Interface Functions:: An abstraction over the basic functions.
+* Basic Functions:: Utility and basic parsing functions.
+* Standards:: A summary of RFCs and working documents used.
+* Index:: Function and variable index.
+@end menu
+
+
+@node Decoding and Viewing
+@chapter Decoding and Viewing
+
+This chapter deals with decoding and viewing @acronym{MIME} messages on a
+higher level.
+
+The main idea is to first analyze a @acronym{MIME} article, and then allow
+other programs to do things based on the list of @dfn{handles} that are
+returned as a result of this analysis.
+
+@menu
+* Dissection:: Analyzing a @acronym{MIME} message.
+* Non-MIME:: Analyzing a non-@acronym{MIME} message.
+* Handles:: Handle manipulations.
+* Display:: Displaying handles.
+* Display Customization:: Variables that affect display.
+* Files and Directories:: Saving and naming attachments.
+* New Viewers:: How to write your own viewers.
+@end menu
+
+
+@node Dissection
+@section Dissection
+
+The @code{mm-dissect-buffer} is the function responsible for dissecting
+a @acronym{MIME} article. If given a multipart message, it will recursively
+descend the message, following the structure, and return a tree of
+@acronym{MIME} handles that describes the structure of the message.
+
+@node Non-MIME
+@section Non-MIME
+@vindex mm-uu-configure-list
+
+Gnus also understands some non-@acronym{MIME} attachments, such as
+postscript, uuencode, binhex, yenc, shar, forward, gnatsweb, pgp,
+diff. Each of these features can be disabled by add an item into
+@code{mm-uu-configure-list}. For example,
+
+@lisp
+(require 'mm-uu)
+(add-to-list 'mm-uu-configure-list '(pgp-signed . disabled))
+@end lisp
+
+@table @code
+@item postscript
+@findex postscript
+Postscript file.
+
+@item uu
+@findex uu
+Uuencoded file.
+
+@item binhex
+@findex binhex
+Binhex encoded file.
+
+@item yenc
+@findex yenc
+Yenc encoded file.
+
+@item shar
+@findex shar
+Shar archive file.
+
+@item forward
+@findex forward
+Non-@acronym{MIME} forwarded message.
+
+@item gnatsweb
+@findex gnatsweb
+Gnatsweb attachment.
+
+@item pgp-signed
+@findex pgp-signed
+@acronym{PGP} signed clear text.
+
+@item pgp-encrypted
+@findex pgp-encrypted
+@acronym{PGP} encrypted clear text.
+
+@item pgp-key
+@findex pgp-key
+@acronym{PGP} public keys.
+
+@item emacs-sources
+@findex emacs-sources
+@vindex mm-uu-emacs-sources-regexp
+Emacs source code. This item works only in the groups matching
+@code{mm-uu-emacs-sources-regexp}.
+
+@item diff
+@vindex diff
+@vindex mm-uu-diff-groups-regexp
+Patches. This is intended for groups where diffs of committed files
+are automatically sent to. It only works in groups matching
+@code{mm-uu-diff-groups-regexp}.
+
+@end table
+
+@node Handles
+@section Handles
+
+A @acronym{MIME} handle is a list that fully describes a @acronym{MIME}
+component.
+
+The following macros can be used to access elements in a handle:
+
+@table @code
+@item mm-handle-buffer
+@findex mm-handle-buffer
+Return the buffer that holds the contents of the undecoded @acronym{MIME}
+part.
+
+@item mm-handle-type
+@findex mm-handle-type
+Return the parsed @code{Content-Type} of the part.
+
+@item mm-handle-encoding
+@findex mm-handle-encoding
+Return the @code{Content-Transfer-Encoding} of the part.
+
+@item mm-handle-undisplayer
+@findex mm-handle-undisplayer
+Return the object that can be used to remove the displayed part (if it
+has been displayed).
+
+@item mm-handle-set-undisplayer
+@findex mm-handle-set-undisplayer
+Set the undisplayer object.
+
+@item mm-handle-disposition
+@findex mm-handle-disposition
+Return the parsed @code{Content-Disposition} of the part.
+
+@item mm-get-content-id
+Returns the handle(s) referred to by @code{Content-ID}.
+
+@end table
+
+
+@node Display
+@section Display
+
+Functions for displaying, removing and saving.
+
+@table @code
+@item mm-display-part
+@findex mm-display-part
+Display the part.
+
+@item mm-remove-part
+@findex mm-remove-part
+Remove the part (if it has been displayed).
+
+@item mm-inlinable-p
+@findex mm-inlinable-p
+Say whether a @acronym{MIME} type can be displayed inline.
+
+@item mm-automatic-display-p
+@findex mm-automatic-display-p
+Say whether a @acronym{MIME} type should be displayed automatically.
+
+@item mm-destroy-part
+@findex mm-destroy-part
+Free all resources occupied by a part.
+
+@item mm-save-part
+@findex mm-save-part
+Offer to save the part in a file.
+
+@item mm-pipe-part
+@findex mm-pipe-part
+Offer to pipe the part to some process.
+
+@item mm-interactively-view-part
+@findex mm-interactively-view-part
+Prompt for a mailcap method to use to view the part.
+
+@end table
+
+
+@node Display Customization
+@section Display Customization
+
+@table @code
+
+@item mm-inline-media-tests
+@vindex mm-inline-media-tests
+This is an alist where the key is a @acronym{MIME} type, the second element
+is a function to display the part @dfn{inline} (i.e., inside Emacs), and
+the third element is a form to be @code{eval}ed to say whether the part
+can be displayed inline.
+
+This variable specifies whether a part @emph{can} be displayed inline,
+and, if so, how to do it. It does not say whether parts are
+@emph{actually} displayed inline.
+
+@item mm-inlined-types
+@vindex mm-inlined-types
+This, on the other hand, says what types are to be displayed inline, if
+they satisfy the conditions set by the variable above. It's a list of
+@acronym{MIME} media types.
+
+@item mm-automatic-display
+@vindex mm-automatic-display
+This is a list of types that are to be displayed ``automatically'', but
+only if the above variable allows it. That is, only inlinable parts can
+be displayed automatically.
+
+@item mm-automatic-external-display
+@vindex mm-automatic-external-display
+This is a list of types that will be displayed automatically in an
+external viewer.
+
+@item mm-keep-viewer-alive-types
+@vindex mm-keep-viewer-alive-types
+This is a list of media types for which the external viewer will not
+be killed when selecting a different article.
+
+@item mm-attachment-override-types
+@vindex mm-attachment-override-types
+Some @acronym{MIME} agents create parts that have a content-disposition of
+@samp{attachment}. This variable allows overriding that disposition and
+displaying the part inline. (Note that the disposition is only
+overridden if we are able to, and want to, display the part inline.)
+
+@item mm-discouraged-alternatives
+@vindex mm-discouraged-alternatives
+List of @acronym{MIME} types that are discouraged when viewing
+@samp{multipart/alternative}. Viewing agents are supposed to view the
+last possible part of a message, as that is supposed to be the richest.
+However, users may prefer other types instead, and this list says what
+types are most unwanted. If, for instance, @samp{text/html} parts are
+very unwanted, and @samp{text/richtext} parts are somewhat unwanted,
+you could say something like:
+
+@lisp
+(setq mm-discouraged-alternatives
+ '("text/html" "text/richtext")
+ mm-automatic-display
+ (remove "text/html" mm-automatic-display))
+@end lisp
+
+Adding @code{"image/.*"} might also be useful. Spammers use images as
+the preferred part of @samp{multipart/alternative} messages, so you might
+not notice there are other parts. See also
+@code{gnus-buttonized-mime-types}, @ref{MIME Commands, ,MIME Commands,
+gnus, Gnus Manual}. After adding @code{"multipart/alternative"} to
+@code{gnus-buttonized-mime-types} you can choose manually which
+alternative you'd like to view. For example, you can set those
+variables like:
+
+@lisp
+(setq gnus-buttonized-mime-types
+ '("multipart/alternative" "multipart/signed")
+ mm-discouraged-alternatives
+ '("text/html" "image/.*"))
+@end lisp
+
+In this case, Gnus will display radio buttons for such a kind of spam
+message as follows:
+
+@example
+1. (*) multipart/alternative ( ) image/gif
+
+2. (*) text/plain ( ) text/html
+@end example
+
+@item mm-inline-large-images
+@vindex mm-inline-large-images
+When displaying inline images that are larger than the window, Emacs
+does not enable scrolling, which means that you cannot see the whole
+image. To prevent this, the library tries to determine the image size
+before displaying it inline, and if it doesn't fit the window, the
+library will display it externally (e.g. with @samp{ImageMagick} or
+@samp{xv}). Setting this variable to @code{t} disables this check and
+makes the library display all inline images as inline, regardless of
+their size.
+
+@item mm-inline-override-types
+@vindex mm-inline-override-types
+@code{mm-inlined-types} may include regular expressions, for example to
+specify that all @samp{text/.*} parts be displayed inline. If a user
+prefers to have a type that matches such a regular expression be treated
+as an attachment, that can be accomplished by setting this variable to a
+list containing that type. For example assuming @code{mm-inlined-types}
+includes @samp{text/.*}, then including @samp{text/html} in this
+variable will cause @samp{text/html} parts to be treated as attachments.
+
+@item mm-text-html-renderer
+@vindex mm-text-html-renderer
+This selects the function used to render @acronym{HTML}. The predefined
+renderers are selected by the symbols @code{w3},
+@code{w3m}@footnote{See @uref{http://emacs-w3m.namazu.org/} for more
+information about emacs-w3m}, @code{links}, @code{lynx},
+@code{w3m-standalone} or @code{html2text}. If @code{nil} use an
+external viewer. You can also specify a function, which will be
+called with a @acronym{MIME} handle as the argument.
+
+@item mm-inline-text-html-with-images
+@vindex mm-inline-text-html-with-images
+Some @acronym{HTML} mails might have the trick of spammers using
+@samp{<img>} tags. It is likely to be intended to verify whether you
+have read the mail. You can prevent your personal informations from
+leaking by setting this option to @code{nil} (which is the default).
+It is currently ignored by Emacs/w3. For emacs-w3m, you may use the
+command @kbd{t} on the image anchor to show an image even if it is
+@code{nil}.@footnote{The command @kbd{T} will load all images. If you
+have set the option @code{w3m-key-binding} to @code{info}, use @kbd{i}
+or @kbd{I} instead.}
+
+@item mm-w3m-safe-url-regexp
+@vindex mm-w3m-safe-url-regexp
+A regular expression that matches safe URL names, i.e. URLs that are
+unlikely to leak personal information when rendering @acronym{HTML}
+email (the default value is @samp{\\`cid:}). If @code{nil} consider
+all URLs safe.
+
+@item mm-inline-text-html-with-w3m-keymap
+@vindex mm-inline-text-html-with-w3m-keymap
+You can use emacs-w3m command keys in the inlined text/html part by
+setting this option to non-@code{nil}. The default value is @code{t}.
+
+@item mm-external-terminal-program
+@vindex mm-external-terminal-program
+The program used to start an external terminal.
+
+@item mm-enable-external
+@vindex mm-enable-external
+Indicate whether external @acronym{MIME} handlers should be used.
+
+If @code{t}, all defined external @acronym{MIME} handlers are used. If
+@code{nil}, files are saved to disk (@code{mailcap-save-binary-file}).
+If it is the symbol @code{ask}, you are prompted before the external
+@acronym{MIME} handler is invoked.
+
+When you launch an attachment through mailcap (@pxref{mailcap}) an
+attempt is made to use a safe viewer with the safest options---this isn't
+the case if you save it to disk and launch it in a different way
+(command line or double-clicking). Anyhow, if you want to be sure not
+to launch any external programs, set this variable to @code{nil} or
+@code{ask}.
+
+@end table
+
+@node Files and Directories
+@section Files and Directories
+
+@table @code
+
+@item mm-default-directory
+@vindex mm-default-directory
+The default directory for saving attachments. If @code{nil} use
+@code{default-directory}.
+
+@item mm-tmp-directory
+@vindex mm-tmp-directory
+Directory for storing temporary files.
+
+@item mm-file-name-rewrite-functions
+@vindex mm-file-name-rewrite-functions
+A list of functions used for rewriting file names of @acronym{MIME}
+parts. Each function is applied successively to the file name.
+Ready-made functions include
+
+@table @code
+@item mm-file-name-delete-control
+@findex mm-file-name-delete-control
+Delete all control characters.
+
+@item mm-file-name-delete-gotchas
+@findex mm-file-name-delete-gotchas
+Delete characters that could have unintended consequences when used
+with flawed shell scripts, i.e. @samp{|}, @samp{>} and @samp{<}; and
+@samp{-}, @samp{.} as the first character.
+
+@item mm-file-name-delete-whitespace
+@findex mm-file-name-delete-whitespace
+Remove all whitespace.
+
+@item mm-file-name-trim-whitespace
+@findex mm-file-name-trim-whitespace
+Remove leading and trailing whitespace.
+
+@item mm-file-name-collapse-whitespace
+@findex mm-file-name-collapse-whitespace
+Collapse multiple whitespace characters.
+
+@item mm-file-name-replace-whitespace
+@findex mm-file-name-replace-whitespace
+@vindex mm-file-name-replace-whitespace
+Replace whitespace with underscores. Set the variable
+@code{mm-file-name-replace-whitespace} to any other string if you do
+not like underscores.
+@end table
+
+The standard Emacs functions @code{capitalize}, @code{downcase},
+@code{upcase} and @code{upcase-initials} might also prove useful.
+
+@item mm-path-name-rewrite-functions
+@vindex mm-path-name-rewrite-functions
+List of functions used for rewriting the full file names of @acronym{MIME}
+parts. This is used when viewing parts externally, and is meant for
+transforming the absolute name so that non-compliant programs can find
+the file where it's saved.
+
+@end table
+
+@node New Viewers
+@section New Viewers
+
+Here's an example viewer for displaying @code{text/enriched} inline:
+
+@lisp
+(defun mm-display-enriched-inline (handle)
+ (let (text)
+ (with-temp-buffer
+ (mm-insert-part handle)
+ (save-window-excursion
+ (enriched-decode (point-min) (point-max))
+ (setq text (buffer-string))))
+ (mm-insert-inline handle text)))
+@end lisp
+
+We see that the function takes a @acronym{MIME} handle as its parameter. It
+then goes to a temporary buffer, inserts the text of the part, does some
+work on the text, stores the result, goes back to the buffer it was
+called from and inserts the result.
+
+The two important helper functions here are @code{mm-insert-part} and
+@code{mm-insert-inline}. The first function inserts the text of the
+handle in the current buffer. It handles charset and/or content
+transfer decoding. The second function just inserts whatever text you
+tell it to insert, but it also sets things up so that the text can be
+``undisplayed'' in a convenient manner.
+
+
+@node Composing
+@chapter Composing
+@cindex Composing
+@cindex MIME Composing
+@cindex MML
+@cindex MIME Meta Language
+
+Creating a @acronym{MIME} message is boring and non-trivial. Therefore,
+a library called @code{mml} has been defined that parses a language
+called @acronym{MML} (@acronym{MIME} Meta Language) and generates
+@acronym{MIME} messages.
+
+@findex mml-generate-mime
+The main interface function is @code{mml-generate-mime}. It will
+examine the contents of the current (narrowed-to) buffer and return a
+string containing the @acronym{MIME} message.
+
+@menu
+* Simple MML Example:: An example @acronym{MML} document.
+* MML Definition:: All valid @acronym{MML} elements.
+* Advanced MML Example:: Another example @acronym{MML} document.
+* Encoding Customization:: Variables that affect encoding.
+* Charset Translation:: How charsets are mapped from @sc{mule} to @acronym{MIME}.
+* Conversion:: Going from @acronym{MIME} to @acronym{MML} and vice versa.
+* Flowed text:: Soft and hard newlines.
+@end menu
+
+
+@node Simple MML Example
+@section Simple MML Example
+
+Here's a simple @samp{multipart/alternative}:
+
+@example
+<#multipart type=alternative>
+This is a plain text part.
+<#part type=text/enriched>
+<center>This is a centered enriched part</center>
+<#/multipart>
+@end example
+
+After running this through @code{mml-generate-mime}, we get this:
+
+@example
+Content-Type: multipart/alternative; boundary="=-=-="
+
+
+--=-=-=
+
+
+This is a plain text part.
+
+--=-=-=
+Content-Type: text/enriched
+
+
+<center>This is a centered enriched part</center>
+
+--=-=-=--
+@end example
+
+
+@node MML Definition
+@section MML Definition
+
+The @acronym{MML} language is very simple. It looks a bit like an SGML
+application, but it's not.
+
+The main concept of @acronym{MML} is the @dfn{part}. Each part can be of a
+different type or use a different charset. The way to delineate a part
+is with a @samp{<#part ...>} tag. Multipart parts can be introduced
+with the @samp{<#multipart ...>} tag. Parts are ended by the
+@samp{<#/part>} or @samp{<#/multipart>} tags. Parts started with the
+@samp{<#part ...>} tags are also closed by the next open tag.
+
+There's also the @samp{<#external ...>} tag. These introduce
+@samp{external/message-body} parts.
+
+Each tag can contain zero or more parameters on the form
+@samp{parameter=value}. The values may be enclosed in quotation marks,
+but that's not necessary unless the value contains white space. So
+@samp{filename=/home/user/#hello$^yes} is perfectly valid.
+
+The following parameters have meaning in @acronym{MML}; parameters that have no
+meaning are ignored. The @acronym{MML} parameter names are the same as the
+@acronym{MIME} parameter names; the things in the parentheses say which
+header it will be used in.
+
+@table @samp
+@item type
+The @acronym{MIME} type of the part (@code{Content-Type}).
+
+@item filename
+Use the contents of the file in the body of the part
+(@code{Content-Disposition}).
+
+@item charset
+The contents of the body of the part are to be encoded in the character
+set specified (@code{Content-Type}). @xref{Charset Translation}.
+
+@item name
+Might be used to suggest a file name if the part is to be saved
+to a file (@code{Content-Type}).
+
+@item disposition
+Valid values are @samp{inline} and @samp{attachment}
+(@code{Content-Disposition}).
+
+@item encoding
+Valid values are @samp{7bit}, @samp{8bit}, @samp{quoted-printable} and
+@samp{base64} (@code{Content-Transfer-Encoding}). @xref{Charset
+Translation}.
+
+@item description
+A description of the part (@code{Content-Description}).
+
+@item creation-date
+RFC822 date when the part was created (@code{Content-Disposition}).
+
+@item modification-date
+RFC822 date when the part was modified (@code{Content-Disposition}).
+
+@item read-date
+RFC822 date when the part was read (@code{Content-Disposition}).
+
+@item recipients
+Who to encrypt/sign the part to. This field is used to override any
+auto-detection based on the To/CC headers.
+
+@item sender
+Identity used to sign the part. This field is used to override the
+default key used.
+
+@item size
+The size (in octets) of the part (@code{Content-Disposition}).
+
+@item sign
+What technology to sign this @acronym{MML} part with (@code{smime}, @code{pgp}
+or @code{pgpmime})
+
+@item encrypt
+What technology to encrypt this @acronym{MML} part with (@code{smime},
+@code{pgp} or @code{pgpmime})
+
+@end table
+
+Parameters for @samp{text/plain}:
+
+@table @samp
+@item format
+Formatting parameter for the text, valid values include @samp{fixed}
+(the default) and @samp{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. @xref{Flowed text}.
+@end table
+
+Parameters for @samp{application/octet-stream}:
+
+@table @samp
+@item type
+Type of the part; informal---meant for human readers
+(@code{Content-Type}).
+@end table
+
+Parameters for @samp{message/external-body}:
+
+@table @samp
+@item access-type
+A word indicating the supported access mechanism by which the file may
+be obtained. Values include @samp{ftp}, @samp{anon-ftp}, @samp{tftp},
+@samp{localfile}, and @samp{mailserver}. (@code{Content-Type}.)
+
+@item expiration
+The RFC822 date after which the file may no longer be fetched.
+(@code{Content-Type}.)
+
+@item size
+The size (in octets) of the file. (@code{Content-Type}.)
+
+@item permission
+Valid values are @samp{read} and @samp{read-write}
+(@code{Content-Type}).
+
+@end table
+
+Parameters for @samp{sign=smime}:
+
+@table @samp
+
+@item keyfile
+File containing key and certificate for signer.
+
+@end table
+
+Parameters for @samp{encrypt=smime}:
+
+@table @samp
+
+@item certfile
+File containing certificate for recipient.
+
+@end table
+
+
+@node Advanced MML Example
+@section Advanced MML Example
+
+Here's a complex multipart message. It's a @samp{multipart/mixed} that
+contains many parts, one of which is a @samp{multipart/alternative}.
+
+@example
+<#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>
+<#/multipart>
+This is a new plain text part.
+<#part disposition=attachment>
+This plain text part is an attachment.
+<#/multipart>
+@end example
+
+And this is the resulting @acronym{MIME} message:
+
+@example
+Content-Type: multipart/mixed; boundary="=-=-="
+
+
+--=-=-=
+
+
+
+--=-=-=
+Content-Type: image/jpeg;
+ filename="~/rms.jpg"
+Content-Disposition: inline;
+ filename="~/rms.jpg"
+Content-Transfer-Encoding: base64
+
+/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRof
+Hh0aHBwgJC4nICIsIxwcKDcpLDAxNDQ0Hyc5PTgyPC4zNDL/wAALCAAwADABAREA/8QAHwAA
+AQUBAQEBAQEAAAAAAAAAAAECAwQFBgcICQoL/8QAtRAAAgEDAwIEAwUFBAQAAAF9AQIDAAQR
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+jQ8KGH1NfQWhXSXmh2c8eArRLwO3HSv/2Q==
+
+--=-=-=
+Content-Type: multipart/alternative; boundary="==-=-="
+
+
+--==-=-=
+
+
+This is a plain text part.
+
+--==-=-=
+Content-Type: text/enriched;
+ name="enriched.txt"
+
+
+<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.
+
+--=-=-=--
+@end example
+
+@node Encoding Customization
+@section Encoding Customization
+
+@table @code
+
+@item mm-body-charset-encoding-alist
+@vindex mm-body-charset-encoding-alist
+Mapping from @acronym{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
+
+@lisp
+((iso-2022-jp . 7bit)
+ (iso-2022-jp-2 . 7bit)
+ (utf-16 . base64)
+ (utf-16be . base64)
+ (utf-16le . base64))
+@end lisp
+
+As an example, if you do not want to have ISO-8859-1 characters
+quoted-printable encoded, you may add @code{(iso-8859-1 . 8bit)} to
+this variable. You can override this setting on a per-message basis
+by using the @code{encoding} @acronym{MML} tag (@pxref{MML Definition}).
+
+@item mm-coding-system-priorities
+@vindex mm-coding-system-priorities
+Prioritize coding systems to use for outgoing messages. The default
+is @code{nil}, which means to use the defaults in Emacs, but is
+@code{(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
+@kbd{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
+@code{(iso-8859-1)}. You can override this setting on a per-message
+basis by using the @code{charset} @acronym{MML} tag (@pxref{MML Definition}).
+
+@item mm-content-transfer-encoding-defaults
+@vindex mm-content-transfer-encoding-defaults
+Mapping from @acronym{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
+@acronym{MIME} encodings, @code{qp-or-base64} may be used to indicate that for
+each case the most efficient of quoted-printable and base64 should be
+used.
+
+@code{qp-or-base64} has another effect. It will fold long lines so that
+MIME parts may not be broken by MTA. So do @code{quoted-printable} and
+@code{base64}.
+
+Note that it affects body encoding only when a part is a raw forwarded
+message (which will be made by @code{gnus-summary-mail-forward} with the
+arg 2 for example) or is neither the @samp{text/*} type nor the
+@samp{message/*} type. Even though in those cases, you can override
+this setting on a per-message basis by using the @code{encoding}
+@acronym{MML} tag (@pxref{MML Definition}).
+
+@item mm-use-ultra-safe-encoding
+@vindex mm-use-ultra-safe-encoding
+When this is non-@code{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).
+
+@end table
+
+@node Charset Translation
+@section Charset Translation
+@cindex charsets
+
+During translation from @acronym{MML} to @acronym{MIME}, for each
+@acronym{MIME} part which has been composed inside Emacs, an appropriate
+charset has to be chosen.
+
+@vindex mail-parse-charset
+If you are running a non-@sc{mule} Emacs, this process is simple: If the
+part contains any non-@acronym{ASCII} (8-bit) characters, the @acronym{MIME} charset
+given by @code{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
+@acronym{MIME} messages.
+@xref{Various Message Variables, , Various Message Variables, message,
+ Message Manual}, for example.)
+If there are only @acronym{ASCII} characters, the @acronym{MIME} charset US-ASCII is
+used, of course.
+
+@cindex MULE
+@cindex UTF-8
+@cindex Unicode
+@vindex mm-mime-mule-charset-alist
+Things are slightly more complicated when running Emacs with @sc{mule}
+support. In this case, a list of the @sc{mule} charsets used in the
+part is obtained, and the @sc{mule} charsets are translated to
+@acronym{MIME} charsets by consulting the table provided by Emacs itself
+or the variable @code{mm-mime-mule-charset-alist} for XEmacs.
+If this results in a single @acronym{MIME} charset, this is used to encode
+the part. But if the resulting list of @acronym{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 @code{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 @acronym{MIME} charset. The part can only be
+split at line boundaries, though---if more than one @acronym{MIME} charset is
+required to encode a single line, it is not possible to encode the part.
+
+When running Emacs with @sc{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
+@code{mm-coding-system-priorities} variable though (@pxref{Encoding
+Customization}).
+
+The charset to be used can be overridden by setting the @code{charset}
+@acronym{MML} tag (@pxref{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
+@code{mm-body-charset-encoding-alist} and
+@code{mm-content-transfer-encoding-defaults} (@pxref{Encoding
+Customization}).
+
+@node Conversion
+@section Conversion
+
+@findex mime-to-mml
+A (multipart) @acronym{MIME} message can be converted to @acronym{MML}
+with the @code{mime-to-mml} function. It works on the message in the
+current buffer, and substitutes @acronym{MML} markup for @acronym{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 @acronym{MML} tags.
+
+@findex mml-to-mime
+An @acronym{MML} message can be converted back to @acronym{MIME} by the
+@code{mml-to-mime} function.
+
+These functions are in certain senses ``lossy''---you will not get back
+an identical message if you run @code{mime-to-mml} and then
+@code{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 @code{mml-to-mime} may decide to use quoted-printable encoding, and
+so on.