mm-coding-system-priorities

mm-coding-system-priorities is a variable defined in `mm-util.el'.
Its value is

nil

Documentation:
Preferred coding systems for encoding outgoing messages.

More than one suitable coding system may be found for some text.
By default, the coding system with the highest priority is used
to encode outgoing messages (see `sort-coding-systems'). If this
variable is set, it overrides the default priority.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.