If they enter their email on the receipt screen that info is not available to access. you can get their email address by enabling email collection https://squareup.com/help/us/en/article/5976-get-started-with-email-collection-tools, or by using the customer directory and adding the customer to the database https://squareup.com/help/us/en/article/5532-manage-your-customer-directory-from-the-square-app.
