Subscriber data should be deleted under several circumstances: 1. Unsubscribe Requests: When a user opts out of your email list, their data should be promptly deleted. 2. Inactive Subscribers: Periodically review and delete data of subscribers who have not engaged with your emails for a certain period, typically 6-12 months. 3. Data Subject Requests: If a subscriber requests the deletion of their data, you are legally obligated to comply.