Over the last few days I have had nothing but trouble using the Sandbox IPN system. At first I thought it was my code, as I had been making changes to our paypal cart system. So I spent several hours rolling back changes, creating a fresh install from a (previously) working codebase, reading through IPN History and listener logs, etc. I descovered that indeed Paypal was sending IPN requests and my listener was successfully receiving them and completing the handshake, but the returned response was returning INVALID from paypal.
So again, I thought it must be my code, or some listener code update that paypal had done that I had not implemented. But I did all that I could to rule that out, comparing my listener to the official paypal PHP example code, adding a missing user agent string to the listener response, ensuring no changes were done since the last time the IPN had been working, etc. And for a few requests here and there it would work, and they would be VALID, but then I would send through more test requests and they would return INVALID.
I'm aware that Paypal has been having some IPN issues over the last few weeks (from the posts I've read on the forum), and I'm wondering if it's possible that the changes that were made to that system indeed fixed IPN messages not being sent, but in the process caused these intermittent INVALID responses
