Outage in Microsoft 365
Some users' mailbox syncs may fail with an error in Exchange Online
Resolved Minor
October 14, 2026 - Started about 6 hours ago
- Lasted about 3 hours
Incident Report
Summary AI Generated
A faulty code change to specific mailbox APIs in Exchange Online caused mailbox sync failures for users whose apps accessed non-Interpersonal Messaging (IPM) mailboxes via Microsoft Graph, returning an "HTTP 400 – Operation not allowed – Operation on Non IPM Subtree forbidden" error. The issue lasted approximately 3.2 hours and affected a subset of users relying on those Graph-based integrations. Microsoft identified the problematic code change and initiated a rollback to remediate the impact.
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.
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.
Components affected
Exchange OnlineTrusted by 1,000+ teams
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Latest Updates ( sorted recent to last )
about 5 hours ago - at 10/14/2026 08:15PM
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
about 6 hours ago - at 10/14/2026 07:13PM
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
about 6 hours ago - at 10/14/2026 07:00PM
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.
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