def failResponse(self, message=b''):
        """
        Send a response indicating failure.

        @type message: stringifyable L{object}
        @param message: An object whose string representation should be
            included in the response.
        """
        if not isinstance(message, bytes):
            message = str(message).encode("utf-8")
        self.sendLine(b'-ERR ' + message)

        Send a response indicating failure.

        @type message: stringifyable L{object}
        @param message: An object whose string representation should be
            included in the response.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/pop3.py