Another (sadly also GPL V3) choice is:

   https://github.com/aaronsw/html2text

which produces markdown from html (given that markdown is safer
than reStructured text it may be a better choice for the conversion).

Then convert to reStructured text (maybe pandoc
--from=markdown --to=rst --output=message.rst message.md
could work.)

In any case, when saved as a file the mime type could be
text/reStructured text and if the libraries are present,
the message could be converted to html.

If anybody decides to do this, make sure to secure
the conversion according to:

 http://docutils.sourceforge.net/docs/howto/security.html