public function batchDelete($listId, array $emails)
    {
        $batchIds = [];
        // as suggested in MailChimp API docs, we send multiple smaller requests instead of a bigger one
        $emailsChunks = array_chunk($emails, self::SUBSCRIBER_BATCH_SIZE);
        foreach ($emailsChunks as $emailsChunk) {
            $Batch = $this->mailchimp->new_batch();
            foreach ($emailsChunk as $index => $email) {
                $emailHash = $this->mailchimp->subscriberHash($email);
                $Batch->delete("op$index", "lists/$listId/members/$emailHash");
            }
            $result = $Batch->execute();
            $currentBatch = $Batch->check_status();
            array_push($batchIds, $currentBatch['id']);
        }
        return $batchIds;
    }