mh-compose-insertion

mh-compose-insertion is a variable defined in `mh-e.el'.
Its value is

mml

Documentation:
Type of tags used when composing MIME messages.

In addition to MH-style directives, MH-E also supports MML (MIME
Meta Language) tags. (see Info node `(emacs-mime)Composing').
This option can be used to choose between them. By default, this
option is set to "MML" if it is supported since it provides a
lot more functionality. This option can also be set to "MH" if
MH-style directives are preferred.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 7.0 of the MH-E package that is part of Emacs 22.1.