A widespread Microsoft outage disrupted Outlook, Exchange Online and parts of Teams on Thursday, 12 November, after unexpected server failures reduced capacity across multiple regions.
Microsoft identified overwhelmed infrastructure as a major factor, but efforts to restore services faltered when restarted servers became unresponsive again.
The disruption continued into Friday, 13 November, affecting businesses and users worldwide. Engineers identified a process handling secure connections that was becoming overwhelmed, although the original cause remained under investigation.
Why Microsoft's Servers Failed
Microsoft initially investigated whether unusually high demand had overwhelmed its infrastructure.
It later found that overall traffic had not increased. Instead, numerous servers unexpectedly became unavailable, placing additional pressure on the remaining machines.
Affected servers failed automated health checks designed to identify malfunctioning equipment. Engineers investigated the component responsible for processing those checks, alongside elevated processor usage.
Microsoft subsequently identified an overwhelmed process responsible for establishing secure connections. This prevented affected servers from handling requests.
Engineers also began reversing a recent network configuration change that coincided with the disruption, although its role had not been confirmed.
Why Microsoft's Fixes Struggled
Engineers restarted servers, redirected connections and tested alternative health checks. These measures initially restored some capacity, allowing limited recovery in certain regions.
However, some restarted servers became unresponsive again when traffic returned. Microsoft responded by restricting certain requests and rebuilding selected machines using an earlier software configuration. Further attempts to stabilise the infrastructure also encountered problems.
Microsoft's Friday updates said a configuration measure was not working as expected, prompting engineers to investigate alternative ways to restore capacity.
How Outlook and Teams Were Affected
Reuters reported that Downdetector recorded more than 130,000 Outlook problem reports in the United States around 1pm Eastern Time on Thursday. These represented submitted reports, not confirmed numbers of affected users.
Exchange Online problems disrupted mailbox access and potentially delayed email delivery. Microsoft said features relying on Exchange Online could also experience problems, although unrelated functions remained operational.
A separate Teams messaging incident affected users in North and South America between 2.45pm and 3.53pm UTC on Thursday.
Microsoft confirmed that the incident had been resolved. It was distinct from the continuing Exchange Online disruption.
Other potentially affected services included SharePoint Online, Microsoft 365 Copilot and Microsoft Defender XDR.
Businesses and AI Connectors Affected
Workhorse Group postponed its third-quarter earnings release and conference call from 12 to 16 November, citing business disruption caused by Microsoft's outage.
OpenAI reported errors affecting its Microsoft 365 connector, including problems establishing new connections and using connected tools.
Anthropic also reported failures involving Microsoft 365 mail, calendar and search tools. Its Claude for Outlook integration was affected.
When Will Microsoft Services Return?
Microsoft's Friday updates described continuing problems with capacity and traffic management. The company said there was no indication that malicious activity caused the disruption.
Although some regions experienced limited recovery, Microsoft had not confirmed the original cause or provided a full restoration time in the updates reviewed. Organisations can monitor Microsoft's service-health dashboard for the latest information.
