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(emacs-mime)Charset Translation

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Charset Translation
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   During translation from MML to MIME, for each MIME part which has
been composed inside Emacs, an appropriate MIME 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.  Note: Various Message Variables,
 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 corresponding MIME charsets are determined.  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, the part is split
into several, 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.