code: 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;
      }
docs: Get the transporter
  
  If no message exists for now, it will be created using the default `transporter` class.
  
  @return string
  @throws \Exception if the default `transporter` class doesn't exist