I found IPN implementation requirements in https://developer.paypal.com/docs/classic/ipn/integration-guide/IPNImplementation/#ipn-listener-request-response-flow There is full URL mentioned
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After implementing of 3 attempt of verification request we changed IPN postback URL to https://ipnpb.paypal.com/cgi-bin/webscr on 15th of July. Since that changes we have no issue with IPN verification at all (all IPN are verified after first attempt)
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We solved this issue by implementing up to 3 attempts IPN verification if it fails. Usually verification successfully completes after second attempt.
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