Customers often fill in an invalid email address on checkout. Its possible to validate the email address in the background. The system sends an SMTP request to the email address and pings if it exists. If it exists, then add a green check-mark to the field. If it fails, then show a notice to the user: 'The billing email address seems incorrect. Please check it.'

Example: https://wpdesk.net/blog/woocommerce-checkout-email-validation/

This will reduce the number of invalid email address, lost sales and customer support requests.