TL;DR
Microsoft 365 remained degraded Friday morning after Exchange Online mailbox access problems began Thursday. Microsoft says a patch is stable on some servers and is being expanded region by region, but it has not identified the root cause.
Microsoft said Friday that a patch had produced stable results on some affected servers and that it was expanding the fix region by region, while its status page continued to list a Microsoft 365 service degradation. The disruption, which began Thursday, has prevented many users from reaching Exchange Online mailboxes and affected other services that depend on it; Microsoft has not named the root cause.
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Microsoft’s first public notice came at 17:54 UTC on November 12, when its Microsoft 365 Status account said it was investigating reports that users in the Americas could not access Exchange Online mailboxes through multiple connection methods. A service health post cited by Cloud Admin Hub listed the incident start as 16:17 UTC. At 18:07 UTC, Microsoft said the problem could affect users in any region.
Microsoft initially described elevated load across multiple regions. It later clarified that the load was not caused by increased traffic: a number of servers had become unexpectedly unavailable, leaving other servers to carry the load. Microsoft said it paused deployments, moved traffic away from affected infrastructure, restarted servers and changed its server health-check method.
Those measures did not keep every returned server stable. In a later update, Microsoft said servers could become unresponsive again after receiving traffic. At 07:29 UTC Friday, it reported positive stability results after applying a patch to part of its infrastructure. Its 08:06 UTC update said the patch was stable where applied and that expansion was proceeding systematically, beginning with regions in or entering peak business hours.
Outage Reaches Connected Work Tools
The disruption extends beyond inbox access because several workplace tools rely on Exchange Online. Microsoft listed effects on Teams calendars and meeting scheduling, parts of SharePoint Online, Microsoft 365 Copilot, the Microsoft Defender XDR portal, Purview and Planner. Users may encounter problems in those products even if the visible failure is not an email screen.
Third-party services were affected as well: OpenAI and Anthropic reported errors in their Microsoft 365 connectors. Workhorse Group said it moved its third-quarter earnings call from November 12 to November 16 because of the outage. These examples show operational consequences for services and organizations that depend on Microsoft 365, though the available reports do not quantify the total business impact.
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From Regional Reports to Wider Impact
The first Microsoft notice referred to mailbox access issues in the Americas. Less than 15 minutes later, the company said the disruption might affect connectivity in any region. CNBC reported that email and other tools were unavailable through much of the Asian business day and into the early European hours.
Reports from Downdetector offer one indication of how many people were reporting trouble, not a verified count of affected users. Reuters, in a report published by The Korea Times, said the site recorded more than 130,000 U.S. Outlook problem reports at the peak, around 1 p.m. ET Thursday. Business Standard reported, citing Downdetector, reports from 74 countries. Neither figure establishes how many people were actually unable to work.
A separate Teams incident, tracked as TM1550488, was declared fully resolved by Microsoft at 17:09 UTC Thursday. BleepingComputer noted that Microsoft had not said whether it was related to the Exchange Online outage. The source material does not establish a connection between the two incidents.
““a number of servers being unexpectedly unavailable””
— Microsoft, in its November 12 service update
Root Cause Remains Unidentified
Microsoft had not identified or published a root cause as of the latest update cited here. Its updates describe unavailable servers, recurring instability under traffic and a patch that was stable in the areas where it had been applied. They do not explain what made the servers unavailable or what caused the core process on affected servers to become overwhelmed.
The status page still listed the incident as service degradation at 08:35 UTC Friday, so a full recovery had not been confirmed at that time. Microsoft said it was not aware of malicious activity and promised a Post-Incident Report after mitigation. The extent of any remaining disruption, the number of affected users and the relationship, if any, to the earlier Teams incident were still unclear.
Patch Expansion and Incident Report
Microsoft’s immediate next step is to continue expanding the patch region by region. The company said it was prioritizing regions in or entering peak business hours, but the cited update did not give a completion time or say when all affected services would be restored.
Readers and affected organizations can follow Microsoft’s service health record EX1550543 for updates. Microsoft said it would publish a Post-Incident Report once the problem is mitigated; that report may provide further detail on the cause and response. Until Microsoft reports recovery, the outage should be treated as ongoing based on the status available at 08:35 UTC.
Key Questions
What is affected by the Microsoft 365 outage?
The incident affects Exchange Online mailbox access through Outlook on the web, desktop and mobile, as well as REST connections. Microsoft also listed effects on Teams calendars and meeting scheduling, parts of SharePoint Online, Copilot, the Defender XDR portal, Purview and Planner.
Has Microsoft fixed the outage?
Not as of 08:35 UTC on November 13. Microsoft said a patch was stable on servers where applied and was being expanded region by region, while its status page still listed service degradation.
What caused the outage?
Microsoft has not stated a root cause. It said some 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.
How many users were affected?
No verified total is given in the source material. Downdetector recorded more than 130,000 U.S. Outlook problem reports at the reported peak, but those are user reports, not a count of affected people.
What happens next?
Microsoft said it would keep expanding the patch and publish a Post-Incident Report once the problem was mitigated. It had not given a restoration deadline in the updates cited here.
Source: Thorsten Meyer AI
