protected function getPrivateKey()
    {
        try {
            $privateKey = file_get_contents($this->privateKeyPath);
        } catch (\Exception $e) {
            $msg = 'Impossible to read the private key of the DKIM swiftmailer signer "%s"';

            throw new RuntimeException(sprintf($msg, $this->privateKeyPath));
        }

        return $privateKey;
    }