code: public function setSpoolDirectory($dir)
      {
          if (!DirectoryHelper::ensureExists($dir)) {
              throw new \Exception(
                  sprintf('Can not create emails spooling directory "%s"!', $dir)
              );
          }
          $this->spool_dir = $dir;
          return $this;
      }
docs: Set the spooled mails directory
  
  @param string $dir The directory where to create spooled mails files
  @return self
  @throws \Exception if the directory doesn't exist and can't be created
  @see \Library\Helper\Directory::ensureExists()