As of June 30, 2017, PayPal Instant Payment Notifications will no longer allow you to use HTTP to post messages back to PayPal for verification. To avoid service disruptions, all merchants using PayPal and Magento 2.0.x must upgrade to Community Edition 2.0.15 by June 30, 2017. All Magento 2.1.x versions already support this change, so no update is required. The new version is available from  the Community Edition Download Page (https://magento.com/tech-resources/download) and more information is available from PayPal at https://www.paypal-knowledge.com/infocenter/index?page=content&widgetview=true&id=FAQ1916&viewlocale=en_US