Magento – PayPal IPN postback failure

This is not a default behavior. The Paypal IPN should advance status forward into the Pending Paypal status. If this status is not available, the results may be unpredictable. I would do the following:

• Check the custom statuses (Admin > System > Order Statuses) and ensure that status for pending_paypal is there and that its state is set to Pending
• Check your Paypal Standard / Express settings and make sure that orders are coming in status Pending.
• Check your IPN endpoint - after 1.4.0.1 it should be http://www.yoursite.com/paypal/ipn/
• File a ticket with Paypal Merchant Technical Support or tweet them (https://twitter.com/PayPal_MTS) to ask them to delay the IPN response by 3-5 minutes (I've had them do this in the past for debugging purposes) to account for a race condition that may be interfering with the order payment / place process

After you've checked all of the above, I would assume a code issue. Disable local modules and follow the standard debugging process.

If your URL is correct then it's what Keyur Shah already commented, but probably wasn't clear enough. If you have IPN enabled and receive payments from different sources (ebay, Magento and others) to the same PayPal account, PayPal unfortunately continues to try sending notifications related to these order to the Magento store, where these orders don't exist.

A possible solution is creating a centralized IPN reciever, see this URL for a very useful script to get you started. http://codeseekah.com/2012/02/11/how-to-setup-multiple-ipn-receivers-in-paypal/