Emails are sent after the trigger conditions you define are met. Say your follow up email is set to trigger based on the purchase of Product X. After a customer purchases Product X and the order status changes to ‘completed,’ WooCommerce Follow Up Emails Extension builds a list of emails to be sent right after the order is completed.
It’s important to note that a follow up email doesn’t get applied retroactively to customers who ‘purchased’ Product X, but rather generates a follow-up email right after the purchase.
If you don’t want multiple emails to be sent to the same customer, check ‘Send once per customer’ for that follow up email.