public function getTransporter()
    {
        if (empty($this->transporter)) {
            $transport_class = $this->getDefault('transporter');
            if (class_exists($transport_class)) {
                $this->setTransporter(new $transport_class);
            } else {
                throw new \Exception(
                    sprintf('Default transport class "%s" not found!', $transport_class)
                );
            }
        }
        return $this->transporter;
    }