We're getting exactly the same annoying message too, in spite of the transactions getting through fine on our end. And those messages are very much annoying, as they do NOT offer any helpful recourse (worse yet: the abominable cop-out boilerplate text of "do not reply to this message" throws you for a loop, sending help that one has to dig deep for) while threatening to disable IPN.
Let me put it this way: because the IPN server does NOT send headers that conform to acceptable standards of traffic, my server has no way of telling PayPal's IPN servers from the riff-raff out there. So, I had to manually whitelist the four IPs to let IPN in, as well as unhooked the URI tickled by IPN from the usual checks for malfeasants.
And in return, now I'm being bombarded with notices that are unhelpful, unresponsive and non-informative.
So, here's my alternative approach: if PayPal can't get its own servers to comply with internet standards by sending appropriate headers, I may disable PayPal myself in favor of another service.
How's that for equity and sanity in customer relations?
Paint me thoroughly un-amused here.
