Microsoft 365 Alert – Service Degradation – Exchange Online – Users may be intermittently unable to access their Exchange Online mailboxes using any connection method – RESOLVED

Announcements Arpan Das

21/11/2025 09:30:00 AM

NHSmail Reference: INC46687690

Microsoft Reference: EX1188132

Issue Status: RESOLVED

Issue Description: Users may be intermittently unable to access their Exchange Online mailboxes using any connection method.

Final Update: 02/12/2025 09:04:00 AM – After a period of monitoring service health, we’ve confirmed that this issue is no longer occurring as a result of our fix deployment that removed the affected indexes and performed a recrawl. We’ve determined from our telemetry that this issue is resolved.

Scope of impact: This issue appears to be intermittently impacting a small subset of users, though potentially any user accessing Exchange Online may be intermittently affected.

Root cause: An indexing issue within a section of mailbox database infrastructure responsible for providing access to Exchange Online mailboxes caused mailbox state invalidations and client disconnections, resulting in impact

Next update by: Microsoft have identified the source of the indexing issue and are developing a solution to prevent issues like this from occurring in the future.