public function subject($subject)
    {
        $name = Config::get('name');
        $subject = '[' . $name . '] ' . $subject;
        /**
         * strange subject encode error
         * occurred when there's both chinese and english
         * solution see: https://github.com/swiftmailer/swiftmailer/issues/665
         */
        $subject = '=?utf-8?B?'.base64_encode($subject).'?=';
        $this->message->setSubject($subject);
        return $this;
    }