I've now received two of these emails from Paypal:
Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs 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.
Checking other posts about this, they seem to relate to IPNs being enabled. IPN haven't been enabled for our Paypal account. Additionally, these emails only started arriving after the MarketPress 2.8.1 update...