This one has popped up a few times in the past. When PayPal has been set-up and configured to with Magento the store owner receives the email below seemingly randomly.
You can check that “Do not receive IPN messages” is set to enabled.
The error appears when the Magento store has been set-up correctly but the store owner also uses PayPal as a payment method on another service. For example eBay, another store etc.
When a payment is made through one of these services PayPal will still try and send a PayPal IPN notification to your store, for an order it didn’t originally process. The notification fails and PayPal think’s something dodgy is up.
Please check your server that handles PayPal Instant Payment Notifications (IPN). Instant Payment Notifications sent to the following URL(s) are failing:
If you do not recognize this URL, you may be using a service provider that is using IPN on your behalf. Please contact your service provider with the above information. If this problem continues, IPNs may be disabled for your account.
Thank you for your prompt attention to this issue.
Yours sincerely, PayPal