public function subscribe()
 {
     $mail_configs['appid'] = $this->appid;
     $mail_configs['appkey'] = $this->appkey;
     if ($this->sign_type != '') {
         $mail_configs['sign_type'] = $this->sign_type;
     }
     $addressbook = new mail($mail_configs);
     return $addressbook->subscribe($this->buildRequest());
 }