Instant Payment Notification (IPN) is PayPal's message service that sends a notification when a transaction is affected. Once IPN is integrated, sellers can automate their back office so they don’t have to wait for payments to come in to trigger order fulfillment.
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.
How It Works
When a customer makes a payment to you or a payment is reversed or refunded, PayPal will post a notification to your server at the URL you specified. Included in this notification will be all of your customer’s payment information (e.g.customer name,payment amount) as well as a piece of encrypted code.When your server receives a notification, it will then post the
information, including the encrypted code, back to a secure PayPal URL.PayPal will authenticate the transaction and send confirmation of its validity back to your server.
The URL for PayPal's IPN responses (www.paypal.com for live and www.sandbox.paypal.com for testing)
try to use www.paypal.com for PayPal's IPN URL first...