if maxheaderlen is
        not specified it defaults to 0, so you must override it explicitly
        if you want a different maxheaderlen.  'policy' is passed to the
        Generator instance used to serialize the message; if it is not
        specified the policy associated with the message instance is used.

        If the message object contains binary data that is not encoded
        according to RFC standards, the non-compliant data will be replaced by
        unicode "unknown character" code points.
        r