PAWsites do not support PayPal IPN.
Some shelters and rescues use PayPal accounts where they have enabled IPN. You can read more about IPN on PayPal's site here:
If your PayPal account has IPN enabled and you set up a donations page on your PAWsite with your PayPal account, PayPal will send you an email that says:
PayPal Instant Payment Notification Warning
Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:
NOTE: This does NOT affect your donations.
Our PAWsites do not support "IPN" aka Instant Payment Notifications from PayPal. You will still get the normal PayPal email notifications they send you when you get a donation. So you can either disable IPN on your PayPal account, or as the email says, you can ignore their warning emails and eventually PayPal will disable IPN for you.
Here's a PayPal help article with the steps to enable or disable IPN on your account:
Please let firstname.lastname@example.org know if you have any other questions!