Some users' mailbox syncs may fail with an error in Exchange Online

Title: Some users' mailbox syncs may fail with an error in Exchange Online User impact: Users' mailbox syncs may have failed with an error in Exchange Online. More info: Users saw the following error message in Microsoft Graph: "HTTP 400 -- Operation not allowed -- Operation on Non IPM Subtree forbidden." Final status: After reverting the recent code change, we've confirmed through service health telemetry that the error code associated with this issue has been resolved. The portion of impacted users who have apps which access non-Interpersonal Messaging (IPM) mailboxes will now see that mailboxes complete syncing as expected. Scope of impact: This issue may have affected some users' mailbox syncs, specifically for customers who use apps that access non-Interpersonal Messaging (IPM) mailboxes through Microsoft Graph, which returned an error code in Exchange Online. Start time: Tuesday, September 29, 2026, at 10:06 PM UTC End time: Wednesday, October 14, 2026, at 8:59 PM UTC Root cause: recent code change intended to improve the resiliency of features associated with IPM mailboxes was enabled across multiple functions, which caused specific apps which rely on these functions being unable to access them. This resulted in the mailboxes of users with these management apps installed failing to sync. Next steps: - We're analyzing the records from this event so that we can ensure that future code updates account for all potential user scenarios. This is the final update for the event.

Title: Some users' mailbox syncs may fail with an error in Exchange Online User impact: Users' mailbox syncs may fail with an error in Exchange Online. More info: Users see the following error message in Microsoft Graph: "HTTP 400 -- Operation not allowed -- Operation on Non IPM Subtree forbidden." Current status: We've identified a recent code change made to specific mailbox APIs which is resulting in the error messages seen in Microsoft Graph when these apps access non-Interpersonal Messaging (IPM) mailboxes. We're reverting this change to verify that it's the source of this issue and to fully remediate impact. Scope of impact: This issue may affect some users' mailbox syncs, specifically for customers using apps that access non-Interpersonal Messaging (IPM) mailboxes through Microsoft Graph, which returns an error code in Exchange Online. This information may be updated as our investigation continues. Next update by: Wednesday, October 14, 2026, at 10:30 PM UTC

Title: Some users' mailbox syncing fails with an error in Exchange Online User impact: Users' mailbox syncing fails with an error in Exchange Online. More info: Users see the following error message in Microsoft Graph: "HTTP 400 -- Operation not allowed -- Operation on Non IPM Subtree forbidden." Current status: We're investigating the error code associated with this impact alongside our service records to identify the source of the issue. Scope of impact: This issue may affect some users' mailbox syncs, specifically for customers using apps that access non-Interpersonal Messaging (IPM) mailboxes through Microsoft Graph, which returns an error code in Exchange Online. This information may be updated as our investigation continues. Next update by: Wednesday, October 14, 2026, at 8:30 PM UTC

Title: Users' mailbox syncing fails with an error in Exchange Online User impact: Users' mailbox syncing fails with an error in Exchange Online. Current status: We're investigating a potential issue with mailbox sync status in Exchange Online and checking for impact to your organization. We'll provide an update within 30 minutes.