Verify callback origin

The recommended approach is using IP firewalling to only allow requests from our trusted IP addresses. We currently use 62.109.57.12, 212.112.190.140, 85.24.146.132 and 176.10.154.199 for IPv4 and 2001:9b0:2:902::199 for IPv6 callbacks.

All customers will be informed via email when these details change.