private function generateEmptyDkimHeader(Message $message)
    {
        // fetch configurable params
        $configurableParams = $this->getParams();

        // check if the required params are set for singing.
        if (empty($configurableParams['d']) || empty($configurableParams['h']) || empty($configurableParams['s'])) {
            throw new \Exception('Unable to sign message: missing params');
        }

        // final params
        $params = array(
            'v'    => $configurableParams['v'],
            'a'    => $configurableParams['a'],
            'bh'   => $this->getBodyHash($message),
            'c'    => 'relaxed',
            'd'    => $configurableParams['d'],
            'h'    => $configurableParams['h'],
            's'    => $configurableParams['s'],
            'b'    => ''
        );

        $string = '';
        foreach ($params as $key => $value) {
            $string .= $key . '=' . $value . '; ';
        }

        // set empty dkim header
        $this->setEmptyDkimHeader(new Dkim(substr(trim($string),0, -1)));
    }