Almost every other web host has paypal as a method for accepting payments. In order to paypal to work smoothly their ips must be whitelisted on the server firewall to ensure that the billing script is notified of the payments correctly. Paypal notifies their ips for sending Instant Payment Notification for the callback url from time to time. Here is the list of latest ips which much be added to firewall on the server on which billing script is located.
As communicated via email in the first week of December 2020, PayPal is expanding the Instant Payments Notification (IPN) infrastructure used to notify merchants about events related to the status of PayPal transactions. This change has added seven new IP addresses from which IPNs are being sent since January 18, 2021. Below are all of the IP addresses that are being used for IPN.
If you are implementing any Access Control List (ACL) or filters on IP addresses for the IPNs received from PayPal, we request that you add all the aforementioned IP addresses before May 1, 2021. Doing so will help avoid missing IPNs from PayPal.