Just found this variable that should fix the problem. Quote
the help on mml-insert-mime-headers-always:

mml-insert-mime-headers-always's 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.

Defined in `mml'.

Looks like, in emacs, solutions just wait there to be discovered.