protected function getPrivateKey()
    {
        if (empty($this->options['private_key_path'])) {
            throw new \RuntimeException('Missing private_key_path for generate a DKIM signature.');
        }

        if (!is_readable($this->options['private_key_path'])) {
            throw new \RuntimeException(sprintf('Private key file does not exists or is not readable: %s', $this->options['private_key_path']));
        }

        $privateKey = file_get_contents($this->options['private_key_path']);

        if (false === $privateKey) {
            throw new \RuntimeException(sprintf('Can\'t read private key in file %s', $this->options['private_key_path']));
        }

        return $privateKey;
    }