public static function send(Swift_Message $message, Swift_SmtpTransport $transport = NULL, Swift_Mailer $mailer = NULL) {
    
    if (!isset($transport) && !isset($mailer)) {
      if (Config::req('mail.smtp.user') != NULL) {
        $transport = Swift_SmtpTransport::newInstance('smtprelaypool.ispgateway.de',465, 'ssl')
          ->setUsername(Config::req('mail.smtp.user'))
          ->setPassword(Config::req('mail.smtp.password'));
      } else {
        $transport = Swift_Mailtransport::newInstance(); // lokaler mailer
      }
    }
  
    if (!isset($mailer)) {  
      $mailer = \Swift_Mailer::newInstance($transport);
    }
    
    return $mailer->send($message);
  }