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I was testing a new module I am writing. It is designed to receive IPN (instant payment notification) postbacks from Paypal. And when the code receives an IPN postback, it in turn calls Paypal again and asks Paypal to verify that the given IPN is real and not fake. Paypal is supposed to look at the IPN data and reply with a string "VERIFIED"
This was working until a few days back, and is not working anymore. After struggling with it for several hours, I found out that Paypal sandbox does not like my code to connect to it in non-SSL mode on port 80. I had replace the plain HTTP connection back to Paypal with HTTPS (SSL) connection on port 443, and everything started working again. From what I read on the Internet, it seems Paypal sandbox has done this several times in the past and then they fix the problem a few days later.