code: public function getSpooler()
      {
          if (empty($this->spooler)) {
              $spool_class = $this->getDefault('spooler');
              if (class_exists($spool_class)) {
                  $this->setSpooler(new $spool_class);
              } else {
                  throw new \Exception(
                      sprintf('Default spool class "%s" not found!', $spool_class)
                  );
              }
          }
          return $this->spooler;
      }
docs: Get the spooler
  
  If no message exists for now, it will be created using the default `spooler` class.
  
  @return string
  @throws \Exception if the default `spooler` class doesn't exist