Paypal Instant Payment Notification Error

I’m using Marketpress with WP Multisite and the first time I attempted to make a purchase,

I received a Paypal error message upon checkout – something about Paypal not being configured properly – and then I got the email below from Paypal. What should I do about this? My site is Thank you!

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

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.