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What is the difference between Delete subscribers and Mass unsubscribe ?
When you delete a subscriber, the subscriber is removed from the database.
When you unsubscribe a subscriber, the subscriber remains in your database but is marked as "unsubscribed".
The added benefit of unsubscribing users is that if you import a new CSV in future and the "unsubscribed" user happens to be in the CSV, they won't be re-imported. The user will remain as "unsubscribed" and no emails can be sent to it.