protected function validateEmailVerification(array $credentials)
    {
        if (is_null($user = $this->getUser($credentials))) {
            return static::INVALID_USER;
        }

        if (! $this->validateToken($user, $credentials)) {
            return static::INVALID_TOKEN;
        }

        if (! $this->validateTimestamp($credentials['expiration'])) {
            return static::EXPIRED_TOKEN;
        }

        return $user;
    }