src/MimeMailer/Mailer.php
protected function spoolMessage($to, $subject, $message, $additional_headers = '', $additional_parameters = '')
    {
        if (empty($this->spool_dir)) {
            throw new \Exception('You must define a spool directory to spool emails!');
        }
        $contents = array(
            'to'=>$to,
            'subject'=>$subject,
            'message'=>$message,
            'additional_headers'=>$additional_headers,
            'additional_parameters'=>$additional_parameters
        );
        return $this->getSpooler()
            ->setSpoolDirectory($this->spool_dir)
            ->addMessageToSpool($this->getMessage()->getId(), $contents, false);
    }
protected function spoolMessage($to, $subject, $message, $additional_headers = '', $additional_parameters = '')
    {
        if (empty($this->spool_dir)) {
            throw new \Exception('You must define a spool directory to spool emails!');
        }
        $contents = array(
            'to'=>$to,
            'subject'=>$subject,
            'message'=>$message,
            'additional_headers'=>$additional_headers,
            'additional_parameters'=>$additional_parameters
        );
        return $this->getSpooler()
            ->setSpoolDirectory($this->spool_dir)
            ->addMessageToSpool($this->getMessage()->getId(), $contents, false);
    }
Messages spooler : prepare the whole content and write it in a file

@param string $to
@param string $subject
@param string $message
@param string $additional_headers
@param string $additional_parameters
@return mixed
@throws \Exception if `spool_dir` is not defined