2.4 Encoding Customization

mm-body-charset-encoding-alist

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).

mm-coding-system-priorities

Prioritize coding systems to use for outgoing messages. The default is nil, which means to use the defaults in Emacs. It is a list of coding system symbols (aliases of coding systems does not work, use M-x describe-coding-system to make sure you are not specifying an alias in this variable). 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-latin-1). You can override this setting on a per-message basis by using the charset MML tag (see section 2.2 MML Definition).

mm-content-transfer-encoding-defaults

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. You can override this setting on a per-message basis by using the encoding MML tag (see section 2.2 MML Definition).

mm-use-ultra-safe-encoding

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).