TL;DR
Microsoft 365 remained degraded at 08:35 UTC on Nov. 13, after an Exchange Online outage disrupted mailbox access and connected services across regions. Microsoft said a patch had stabilized servers where applied and was being expanded region by region; the root cause remains unknown.
Microsoft’s Microsoft 365 outage was still ongoing at 08:35 UTC on Nov. 13, with the company reporting service degradation after an Exchange Online failure disrupted mailbox access and related services across regions. Microsoft said a patch had produced stable servers where applied and was being expanded region by region, but it had not identified the outage’s root cause.
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The incident, tracked as EX1550543, began at 16:17 UTC on Nov. 12, according to a copy of Microsoft’s service health post cited by Cloud Admin Hub. Microsoft’s first public status update came at 17:54 UTC, saying it was investigating reports that users in the Americas could not access Exchange Online mailboxes through multiple connection methods. At 18:07 UTC, it said the issue might affect Microsoft 365 connectivity in any region.
Microsoft initially described elevated load across multiple regions as a likely contributor. 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 more load. The company paused deployments, shifted traffic, restarted servers and changed its method for checking server health. Those steps did not fully resolve the problem; Microsoft said servers that returned to service could become unresponsive again once they received traffic.
At 07:29 UTC on Nov. 13, Microsoft reported positive server stability results after applying a patch to part of its infrastructure. In an 08:06 UTC update, it said servers were stable where the patch had been applied and that expansion was proceeding systematically, beginning with regions in or entering peak business hours. The service status remained degraded at 08:35 UTC.
How the Outage Reached Other Services
The disruption extended beyond Exchange Online email. Users also faced problems with Outlook on the web, desktop and mobile, as well as REST connections. Services that rely on Exchange Online—including Teams calendars and meeting scheduling—were affected, alongside parts of SharePoint Online, Microsoft 365 Copilot, the Microsoft Defender XDR portal, Purview and Planner.
The outage also reached third-party services and business operations. OpenAI and Anthropic reported errors with their Microsoft 365 connectors. Workhorse Group said it moved its third-quarter earnings call from Nov. 12 to Nov. 16 because of the outage. These effects show how a failure in a shared cloud service can disrupt tools and schedules beyond email, though the specific impact on each organization was not detailed in the supplied reports.
Reports from Downdetector indicate the scale of user complaints, but they are not a count of people affected. Reuters, in a report published by The Korea Times, said the site recorded more than 130,000 Outlook problem reports in the United States at the peak, around 1 p.m. ET on Nov. 12. Business Standard reported complaints from 74 countries, citing Downdetector. Those figures describe reports submitted to the service, not verified outage totals.
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From Americas Reports to Regional Patching
Microsoft’s status updates trace the incident from an initial problem accessing mailboxes in the Americas to connectivity issues that could affect users in any region. At 18:35 UTC on Nov. 12, the company said a portion of its infrastructure was experiencing elevated load across multiple regions. By 19:31 UTC, it said the load stemmed from unexpectedly unavailable servers rather than a rise in traffic.
Microsoft’s response shifted as its initial recovery steps proved temporary. The company said returned servers could become unresponsive again under traffic. Its later update reported improved stability with a patch and described a region-by-region expansion. According to CNBC, email and other tools were unavailable through much of the Asian business day and into the early European hours.
A separate Teams incident, TM1550488, was declared fully resolved by Microsoft at 17:09 UTC on Nov. 12. BleepingComputer reported that Microsoft had not said whether that incident was related to EX1550543. The available status information does not establish a connection between them.
““A portion of infrastructure is experiencing elevated load across multiple regions, which is likely contributing to the connectivity errors.””
— Microsoft 365 Status account, Nov. 12, 18:35 UTC
Root Cause and Recovery Scope
Microsoft has not named the root cause of the server unavailability or explained why the affected servers became unresponsive. The company said it was “not aware of any malicious activity,” which describes its current assessment rather than a final finding. It has promised a Post-Incident Report after the problem is mitigated.
As of 08:35 UTC, Microsoft had not said when the patch expansion would reach all affected regions or when services would return to normal. The available updates also do not quantify how many organizations or users were affected, or confirm whether every listed connected service experienced the same symptoms. Downdetector figures are reports from users, not verified counts of affected people.
Microsoft’s Regional Patch Rollout
Microsoft said it would continue expanding the patch region by region, starting with areas in or approaching peak business hours. The company’s status page and the EX1550543 service record remain the sources cited for updates on service health. The outage should be considered ongoing until Microsoft reports mitigation or restoration.
After mitigation, Microsoft has said it will publish a Post-Incident Report. That report is expected to provide further information about the root cause and response; no publication time was given in the supplied material.
Key Questions
Is the Microsoft 365 outage over?
No. Microsoft’s status page still listed service degradation at 08:35 UTC on Nov. 13. The company was expanding a patch at that time.
Which services were affected?
The outage disrupted access to Exchange Online mailboxes through Outlook on the web, desktop and mobile, and REST connections. Reported effects also included Teams calendars and meeting scheduling, parts of SharePoint Online, Copilot, the Defender XDR portal, Purview and Planner.
What caused the outage?
Microsoft has not identified the root cause. It said some servers became unexpectedly unavailable, which increased load on others, and that returned servers could become unresponsive again under traffic.
Did Microsoft report a cyberattack?
Microsoft said it was not aware of malicious activity. That is the company’s current statement; it has not yet published its promised Post-Incident Report.
When will Microsoft 365 return to normal?
Microsoft had not provided a restoration time in the updates summarized here. It said the patch was stable where applied and was being expanded region by region.
Source: Thorsten Meyer AI
