code: protected function saveMessage($message)
      {
          $path = Yii::getAlias($this->fileTransportPath);
          if (!is_dir($path)) {
              mkdir($path, 0777, true);
          }
          if ($this->fileTransportCallback !== null) {
              $file = $path . '/' . call_user_func($this->fileTransportCallback, $this, $message);
          } else {
              $file = $path . '/' . $this->generateMessageFileName();
          }
          file_put_contents($file, $message->toString());
  
          return true;
      }
docs: Saves the message as a file under [[fileTransportPath]].
  @param MessageInterface $message
  @return bool whether the message is saved successfully