code: func checkResponseError(resp *http.Response) error {
  	if resp.StatusCode < 200 || resp.StatusCode >= 400 {
  		var mailjetErr RequestError
  		err := json.NewDecoder(resp.Body).Decode(&mailjetErr)
  
  		if err != nil {
  			return fmt.Errorf("Unexpected server response code: %d: %s", resp.StatusCode, err)
  		}
  		return fmt.Errorf("Unexpected server response code: %d: %s (%s)",
  			resp.StatusCode, mailjetErr.ErrorMessage, mailjetErr.ErrorInfo)
  	}
  	return nil
  }
docs: // checkResponseError returns response error if the statuscode is < 200 or >= 400.