In Sept 2016 PayPal are changing their requirement for posting IPN messages back to PayPal for verification.
They are making the post url as https. My current versions of scripts already post to https so there should not be any change required, however, they also suggest that https://ipnpb.paypal.com/cgi-bin/webscr should be used in the future rather than https://www.paypal.com/cgi-bin/websc
Change:
https://www.paypal.com/cgi-bin/webscr
to
https://ipnpb.paypal.com/cgi-bin/webscr