Instant Payment Notification (IPN) is a PayPal message service that notifies you of events related to your PayPal transactions. You can use it to automate office and administrative functions, such as fulfilling orders, tracking customers, and providing other information related to a transaction.
You can also:
Resend messages not received by your website's back-end systems.
Temporarily stop receiving messages (useful when performing maintenance on your back-end systems).
Messages are generated but stored at PayPal until you start receiving them again.
Use the IPN feature in these situations:
Your service provider, cart provider or website developer has asked you to.
You have back-end systems that require IPN messages to automate business processes, such as creating shipping notifications and inputs to accounting applications.