protected function sendLater($sender, $receiver, $subject, $message, array $headers = []) {
        if (!empty($this->options['send-later'])) {
            $callable = [$this, 'sendMail'];
            register_shutdown_function($callable, $sender, $receiver, $subject, $message, $headers);

            $this->options['send-later'] = false;
            return true;
        }
        return false;

        // TODO: Not yet found a way to send a "Location" header (redirect) to the client, close the browser connection
        //       and keep the mail script sending in background with "output_buffering" enabled. As the output buffer is
        //       never full from just a redirect header PHP is waiting for the shutdown function to finish as it might
        //       push more content into the buffer. Maybe "output_buffering" can be disabled when entering shutdown?
    }