public function isValid()
    {
        if (!filter_var($this->email, FILTER_VALIDATE_EMAIL)) {
            return false;
        }

        if (empty($this->userAgent)) {
            return false;
        }

        if (empty($this->series)) {
            return false;
        }

        if (empty($this->token)) {
            return false;
        }

        return true;
    }