Thank you for posting to the PayPal Sandbox Community.
I tested the IPN Simulator in Sandbox using https://webhook.site it is a website that you can use for IPN and Webhook simulation.
IPNs are being sent with no issues.
Here is the information on implementing a listener: https://developer.paypal.com/docs/ipn/integration-guide/IPNImplementation/
Your listener must send the post back in an https request.
Here is the information on IPN Architecture:
Here is a basic troubleshooting guide to receiving 404 errors with the IPN Listener:
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