code: public function send(\Zend\Mail\Message $message)
      {
          $options  = $this->options;
          $filename = call_user_func($options->getCallback(), $this);
          $file     = $options->getPath() . DIRECTORY_SEPARATOR . $filename;
  
  
          $contents = $message->toString();
          $umask = umask();
          umask(022);
          if (false === file_put_contents($file, $contents, LOCK_EX)) {
              throw new RuntimeException(sprintf(
                  'Unable to write mail to file (directory "%s")',
                  $options->getPath()
              ));
          }
          umask($umask);
          $this->lastFile = $file;
      }
docs: Saves e-mail message to a file
  
  @param \Zend\Mail\Message $message
  @throws RuntimeException on not writable target directory or
  on file_put_contents() failure