mml-insert-mime-headers-always

mml-insert-mime-headers-always is a variable defined in `mml.el'.
Its value is

t

Documentation:
If non-nil, always put Content-Type: text/plain at top of empty parts.
It is necessary to work against a bug in certain clients.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.