PayPal IPN URL failing?

We received the following email from PayPal 2 days in a row:

Please check your server that handles PayPal Instant Payment Notifications (IPN). Instant Payment Notifications sent to the following URL(s) are failing:

http://(domain).com/paymentreturn/paypalexpress

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.

How can I check this is all OK?