PayPal Instant Payment Notification Warning

I am constantly getting an email from paypal with the content

PayPal Instant Payment Notification Warning:

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,

I have checked the settings on paypal end and I have the correct address in.

I know this is fairly a common thread but I haven't seen anything that helps.
Any ideas?