PayPal Instant Payment Notifications (IPN) failing

I use palpal to process payments from Events+ plugin that requires nothing save for my email.

Paypal is sending my client this notification everyday. Please advise:

Subject: PayPal Instant Payment Notification Warning

Hello Robert R,

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.