What is an IPN?
Instant Payment Notification (IPN) is a message service that automatically notifies merchants of events related to PayPal transactions. Merchants can use it to automate back-office and administrative functions, including automatically fulfilling orders and providing customers with order status.
PayPal IPN (Instant Payment Notification) allows you to integrate your PayPal payments with your website’s back-end operations, so you get immediate notification and authentication of the PayPal payments you receive.
To enable PayPal IPN please follow the following steps:
http://your-doamin.com/?action=wpdmpp-payment-notification&class=Paypalfor that field.