PayPal Instant Payment Notification Warning

I’m receiving these emails from paypal.

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.

Thank you for your prompt attention to this issue.

I checked my IPN setup at Paypal and the Notification URL is set to: That is my other site

How to fix this and have multiple sites?