By default, the Preorder email is off.
What the customer is normally receiving is the order confirmation from Shopify.

Simply enable the "Preorder Emails" feature from the Settings tab of the app. You can customize the email templates from that tab as well. The cost for email sending is $1 for 100 emails.

If you enable our app's email add-on, your customer will receive both the normal email from Shopify and the pre-order email.
A pre-order confirmation email is a separate email that gets sent to your customers after they order a pre-order product.
You can include details like your pre-order product's expected shipping date in this email.