IPN Error Messages from PayPal Express

We received two warning messages from PayPal Express on our site which seems to be working perfectly; orders are made, processed by PayPal and paid, email notices sent and orders appearing in Products => Manage Orders. Here is the latest email warning from PayPal:

SUBJECT: PayPal Instant Payment Notification Disabled
Hello Glenfinnan Music Ltd,

Please check your server that handles PayPal Instant Payment Notification (IPN) messages. Messages sent to the following URL(s) are not being received:

http://www.alexbeaton.com/store/payment-return/paypal-express

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.

Once you or your service provider fix this problem, you or your service provider can resend the failed messages from the IPN History page. If this problem continues, PayPal may disable the IPN feature for your account.

Thank you for your prompt attention to this issue.
Thanks,
PayPal

So, is there something I need to set in the software? I see nothing in the Products => Manage for the Marketpress plugin.

Thanks in advance for your guidance.
Merv (@lzw8bk)