Followup: We started successfully receiving IPN messages again on 2/19 and things have been working fine since. PayPal Support finally responded to our support ticket a day and a half after we submitted it, but by then things were working again, so they did not offer any real help. From our postmortem review on this issue, we noticed that our IPN Notification URL was set up in PayPal as 'http://...', so we changed this to 'https://...', and all IPN messages from this point on were successfully sent to us (200). We do not know with high confidence that this change fixed the issue as it may have been coincidental, but there you are. If in fact, PayPal made a change that all Notification URL's must now be to 'https://...', obviously, that would have been highly beneficial to know in advance,
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