How to fix “PayPal Instant Payment Notification (IPN) messages”

Your got message with the following text:

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Please check your server that handles PayPal Instant Payment Notification (IPN) messages. Messages sent to the following URL(s) are not being received:
 

http://yourdomain.com/url


If you do not recognize this URL, you may be using a service provider that is using IPN on your behalf.… Read the rest