TL;DR
Microsoft 365 remained degraded at 08:35 UTC on Nov. 13, with users reporting trouble reaching Exchange Online mailboxes and related services. Microsoft said a patch had stabilized servers where applied and was being expanded region by region; the company had not identified the root cause.
In its 08:06 UTC update to incident EX1550543, Microsoft said servers were stable in areas where it had applied a patch and that it had started expanding the patch region by region, prioritizing regions in or entering peak business hours. At 07:29 UTC, the company had reported positive results from applying the patch to part of its infrastructure. Its status page still listed Microsoft 365 as experiencing “Service degradation” as of 08:35 UTC.
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The incident affects access to Exchange Online mailboxes through Outlook on the web, desktop and mobile, as well as REST connections, according to the source material. Reported knock-on effects include Teams calendars and meeting scheduling, parts of SharePoint Online, Microsoft 365 Copilot, the Microsoft Defender XDR portal, Purview and Planner. OpenAI and Anthropic also reported errors in their Microsoft 365 connectors. The available updates do not establish that every user or feature of these services is affected.
Microsoft’s status updates traced the problem to servers becoming unexpectedly unavailable across multiple regions. The company paused deployments, shifted traffic, restarted servers and changed its method for checking server health. It later said a core process on affected servers was becoming overwhelmed, and that some restarted servers became unresponsive again after receiving traffic. Microsoft has said it is not aware of malicious activity; that statement does not identify the underlying cause.
Disruptions Spread Beyond Outlook
The outage reaches beyond email because several workplace tools depend on Exchange Online for calendars, scheduling or related functions. When mailbox access or those connected features fail, employees may have difficulty coordinating meetings and accessing parts of their usual Microsoft 365 workflow. The specific impact will vary by user and service.
The disruption has also affected organizations outside Microsoft’s own apps. OpenAI and Anthropic reported errors in their Microsoft 365 connectors, while Workhorse Group moved its third-quarter earnings call from Nov. 12 to Nov. 16, citing the outage. These examples show practical consequences, but they do not establish the full number of organizations or users affected.
Public outage trackers offer a rough measure of complaints, not a count of people unable to work. Reuters, as published by The Korea Times, reported more than 130,000 U.S. Outlook problem reports on Downdetector at the peak around 1 p.m. ET Thursday; Business Standard cited reports spanning 74 countries. Those figures are user reports, and the source material provides no verified total of affected users.
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How the Outage Developed
Microsoft’s first public notice came at 17:54 UTC on Nov. 12, when its Microsoft 365 Status account said it was investigating reports of Exchange Online mailbox access problems through multiple connection methods in the Americas. The incident record says the issue began at 16:17 UTC. At 18:07 UTC, Microsoft said the problem might affect connectivity for users in any region.
At 18:35 UTC, Microsoft described elevated load across multiple regions. It later clarified that the load was not caused by increased traffic but by servers becoming unexpectedly unavailable. The company said it had paused deployments, redistributed traffic and restarted servers. By 21:00 UTC, it had changed its server health-check method and returned some servers to service; an update near midnight said returned servers could become unresponsive again under traffic.
A separate Teams incident, TM1550488, was marked fully resolved at 17:09 UTC on Nov. 12. BleepingComputer reported that Microsoft had not said whether it was related to EX1550543. The available source material does not establish a connection between the two incidents.
“Servers are stable where the patch has been applied.”
— Microsoft, in its 08:06 UTC service health update for EX1550543
Cause and Recovery Remain Open
The root cause has not been disclosed. Microsoft’s description of an overwhelmed core process identifies a symptom or technical condition, but the updates in the source material do not explain why it occurred. The company has not said how long the regional patch expansion will take or when all affected services will return to normal.
It is also unclear how many users and organizations have been affected, which regions or individual features remain impaired, and whether the separate Teams incident was connected. Downdetector figures reflect reports rather than verified affected-user counts. Microsoft says it is not aware of malicious activity, but has not published a final investigation finding.
Watch Microsoft’s Regional Rollout
Microsoft said it was expanding the patch region by region, starting with areas in or entering peak business hours. Users and administrators should consult Microsoft’s service health updates for EX1550543 to follow the rollout and check whether the services they rely on have recovered. The source material does not provide a confirmed completion time.
Microsoft has promised a Post-Incident Report after the problem is mitigated. That report is expected to provide more information about the cause and response, but its publication date and the details it will contain have not been specified. The outage should be treated as ongoing until Microsoft reports that service has recovered.
Key Questions
Is the Microsoft 365 outage over?
No. As of 08:35 UTC on Nov. 13, Microsoft’s status page still listed a service degradation. The company said a patch was stabilizing servers where applied and was being expanded region by region.
Which services have been affected?
The reported disruptions include Exchange Online mailbox access through Outlook and REST connections, along with effects on Teams calendars and meeting scheduling, parts of SharePoint Online, Copilot, Defender XDR, Purview and Planner. The source does not say every user or feature is affected.
What caused the outage?
Microsoft has not named the root cause. It said servers became unexpectedly unavailable and that a core process on affected servers was becoming overwhelmed. The company said it was not aware of malicious activity.
What should users do now?
Check Microsoft’s service health updates for incident EX1550543 for current status. Microsoft had not provided an estimated time for full recovery in the updates cited here.
When will Microsoft publish more detail?
Microsoft has promised a Post-Incident Report after the issue is mitigated. It has not announced when that report will appear.
Source: Thorsten Meyer AI
