As of September 1, 2017, Shopify will begin verifying SSL certificates when delivering webhook payloads to HTTPS web hook addresses.
After this date, webhooks that fail the SSL certificate verification will not successfully deliver to their hosts.
Please ensure your server is correctly configured to support HTTPS with a valid server certificate. We suggest using a SSL checker to help you diagnose any problems with your SSL certificate installation.
If you have any questions or concerns, please feel free to reply in the thread below.
All the best,