Using Spently's follow-up emails allows you to engage with a shopper after they have purchased from you. To send follow-ups for orders that occurred before the follow-up email was set live, you will have to sync your past purchases.
You can only sync previous purchases on the Essential Plan and above. Simply click "Sync." when you are setting up an auto follow-up. You will receive an email once this process is complete. You do not need to do this for each email as this will apply to all off your auto follow-up emails.