An Office service that supports add-ins to interact with objects in Office client applications.
The behavior described suggests the issue may be related to network configuration rather than the Microsoft Graph subscription API itself. Since allowing additional inbound IP addresses resolved the issue, you may want to review the documented Microsoft Graph Change Notifications IP address ranges for your cloud environment rather than relying on individual IP addresses observed from Bruno.
Receive change notifications through webhooks - Microsoft Graph notes that firewalls can be configured to allow inbound connections from Microsoft Graph change notification services. If any firewall, WAF, reverse proxy, or IP restriction is present in front of the URLs used for Microsoft Graph change notifications, the documented ranges may be more appropriate than maintaining a list of individual addresses captured during testing. Individual IPs observed from Bruno may not represent all Microsoft Graph service endpoints and could change over time.
You may also find the following discussion relevant, as it describes the same validation-related error: Microsoft Graph Subscription Validation Fails with 403 Forbidden Despite Reachable Notification End…
I hope this information is helpful.
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