PayPal IPN problem

I just got an email with the message below. The only things I have on PopCred.net that are connected to PayPal are MarketPress and Prosites. I have numerous MarketPress sales and some Pro Sites subscriptions.

Help?

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

http://popcred.net/wp-admin/admin-ajax.php?action=psts_pypl_ipn

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.

Thanks,

PayPal