We've been running Emerald 9.428 on Joomla 3.6.2 without a problem, but just received the notice from PayPal (see image).
Not sure what this means or how to fix the problem. Thank you
Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs 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.