Live status of all services Incident · resolved

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

Exchange Online EX1507783

Users' mailbox syncs may have failed with an error in Exchange Online.

Status
Service restored
Feature
Outlook - poor customer experience
Region
Not specified (all regions)
Started
Sep 30, 2026, 00:06 CEST
Resolved
Oct 14, 2026, 23:43 CEST
Duration
14 days 23 h

More info

Users saw the following error message in Microsoft Graph: "HTTP 400 -- Operation not allowed -- Operation on Non IPM Subtree forbidden."

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.

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.

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.

Timeline

Every update Microsoft posted for this issue, newest first.

  1. Oct 14, 2026, 23:44 CEST

    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.

  2. Oct 14, 2026, 22:15 CEST

    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.

  3. Oct 14, 2026, 21:13 CEST

    We're investigating the error code associated with this impact alongside our service records to identify the source of the issue.

  4. Oct 14, 2026, 21:00 CEST

    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.

When Microsoft 365 goes down

The live overview shows the current state of every Microsoft 365 service, refreshed every minute from Microsoft Graph.

Live status of all services Subscribe to incidents in LEXOJ

To see whether your own tenant is affected, check Service health in the Microsoft 365 admin center. Only a backup outside Microsoft protects against data loss, and only a tested break-glass account against locked-out admins. We set up both in fixed-scope sprints.

Source: Microsoft Graph, Service Communications API. This is the view of a Swiss Microsoft 365 tenant: services not licensed there are missing, and individual issues may affect your tenant differently. Microsoft names the region only in the issue text. The Switzerland view shows issues for Switzerland, for Europe in general and those without a region, but not issues that only name other countries such as Germany or the UK. This page is not provided by Microsoft.