Send a raw MIME message.

def raw(self, from_, to, body):
        """
        Send a raw MIME message.
        """
        if isinstance(to, string_types):
            raise TypeError('"to" parameter must be enumerable')
        return self._session.post('{}/raw'.format(self._url), json={
            'from': from_,
            'to': to,
            'body': body,
        }).json()