TL;DR
Microsoft 365 remained degraded Friday morning after Exchange Online mailbox access problems began Thursday. Microsoft says a patch has stabilized servers where applied and is expanding region by region; it has not disclosed the root cause.
Microsoft 365 was still experiencing a service degradation as of 08:35 UTC Friday, November 13, after Exchange Online problems that began Thursday disrupted mailbox access and connected services. In an 08:06 UTC update, Microsoft said servers were stable where a patch had been applied and that it was expanding the patch region by region; the company had not named the underlying cause.
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The incident, tracked as EX1550543, affects access to Exchange Online mailboxes through Outlook on the web, desktop and mobile apps, as well as REST connections, according to the source material. The disruption has also affected services that depend on Exchange Online, including Teams calendars and meeting scheduling, some SharePoint features, Microsoft 365 Copilot, the Microsoft Defender XDR portal, Purview and Planner.
Microsoft’s updates describe a changing response. It first cited elevated load across multiple regions, then said the load was not caused by increased traffic but by servers that had become unexpectedly unavailable. Microsoft paused deployments, redistributed traffic and restarted servers. It also changed its server health-check method and said some servers returned to service.
Those restarts did not fully resolve the problem. Microsoft later reported that returned servers could become unresponsive again once they received traffic. At 07:29 UTC Friday, it reported positive stability results after applying a patch to part of its infrastructure. At 08:06 UTC, it said it had begun expanding the patch systematically, starting with regions in or approaching peak business hours. The status page continued to list the incident as service degradation.
How the Outage Disrupted Work
The effects extend beyond people unable to open email. When Exchange Online is unavailable or unstable, connected tools can lose access to calendars, scheduling and other mailbox-dependent functions. That can interrupt routine coordination across organizations, even when some separate Microsoft 365 features remain available.
The outage has also affected third-party integrations. OpenAI and Anthropic reported errors with their Microsoft 365 connectors, according to their status pages. Workhorse Group said it moved its third-quarter earnings call from November 12 to November 16 because of the outage. These examples show concrete effects on business workflows, though the source material does not establish how many organizations or users were affected overall.
Reports on Downdetector provide an indication of the volume of complaints, not a verified count of people without service. Reuters, as published by The Korea Times, reported more than 130,000 U.S. Outlook problem reports at the Thursday peak, around 1 p.m. ET. Business Standard, citing Downdetector, reported reports from 74 countries. Those figures should be read as user-submitted reports, not as confirmed totals of affected users.
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From Thursday Reports to Friday Patching
Microsoft’s first public notice came at 17:54 UTC Thursday, November 12. The Microsoft 365 Status account on X said it was investigating reports of users having trouble accessing Exchange Online mailboxes through multiple connection methods in the Americas. A copy of the service-health notice on Cloud Admin Hub lists the incident start as 16:17 UTC.
At 18:07 UTC, Microsoft said the issue might affect Microsoft 365 connectivity in any region. Later updates described elevated load, traffic redistribution and unavailable servers. By 23:59 UTC, Microsoft had warned that servers brought back online could become unresponsive again under traffic. The move to patching marked a new phase in the response, but Microsoft’s Friday morning updates still described an active incident.
A separate Teams incident, tracked as TM1550488, was declared fully resolved at 17:09 UTC Thursday. BleepingComputer reported that Microsoft had not said whether that earlier issue was related to EX1550543. The available information does not establish a connection between them.
““a number of servers being unexpectedly unavailable””
— Microsoft, in its November 12 status updates
Cause and Recovery Remain Open
Microsoft has not identified the root cause in the updates described here. Its statement that it is not aware of malicious activity is not a finding about what caused the failure. The company said it would publish a Post-Incident Report after mitigation.
It was also unclear at the 08:35 UTC status check how quickly the patch would reach all affected regions, when mailbox access and connected services would return to normal, or whether users would experience further interruptions during the rollout. Microsoft’s report of stable servers applies to locations where the patch had been applied; it does not establish that the wider service had recovered.
The available reports do not give a confirmed count of affected users or organizations. Downdetector figures reflect submitted reports, and the information provided does not specify a verified total or a complete measure of service impact.
Microsoft’s Regional Patch Rollout
Microsoft said it was expanding the patch region by region, beginning with areas in or entering peak business hours. Its next updates will need to clarify whether the patched servers remain stable under traffic and whether the rollout restores access across Exchange Online and connected Microsoft 365 services.
For now, affected users can check Microsoft’s service-health information for the current status of EX1550543 and follow their organization’s IT guidance. The source material does not describe a confirmed workaround that restores mailbox access. Microsoft said it would publish a Post-Incident Report once the incident is mitigated; that report may provide a fuller account of the cause and response.
Key Questions
Is the Microsoft 365 outage over?
No. As of 08:35 UTC on November 13, Microsoft’s status page still listed service degradation. Microsoft had reported stability where its patch was applied and was expanding the rollout.
Which services are affected?
The reported issue centers on Exchange Online mailbox access through Outlook and REST connections. Connected services listed in the source material include Teams calendars and scheduling, parts of SharePoint, Copilot, the Defender XDR portal, Purview and Planner.
What caused the outage?
The root cause has not been disclosed. Microsoft said the elevated load was not caused by more traffic and linked it to unexpectedly unavailable servers. It also said it was not aware of malicious activity.
What should affected users do?
Check Microsoft’s service-health updates for incident EX1550543 and follow guidance from your organization’s IT team. The available source material does not confirm a workaround that restores mailbox access.
When will Microsoft explain what happened?
Microsoft said it would publish a Post-Incident Report after the problem is mitigated. The source material does not give a publication date.
Source: Thorsten Meyer AI
