public function synchronize(SubscriberListInterface $list)
    {
        $listData = $this->listRepository->findById($list->getListId());

        // get Subscribers from the provider
        $subscribers = $list->getProvider()->getSubscribers();

        // unsubscribe difference
        $this->unsubscribeDifference($list->getListId(), $subscribers);
        // subscribe the rest
        return $this->batchSubscribe($list->getListId(), $subscribers);
    }