A broad Microsoft 365 outage disrupted Outlook access and email delivery from Thursday, November 12, into Friday morning. Microsoft reported partial recoveries overnight, but the initial mitigations did not keep services stable, leaving users with intermittent access to mailboxes and delayed messages.
The incident centers on Exchange Online, Microsoft’s cloud platform for email, calendars and contacts. Its disruption affected several Outlook access methods and related Microsoft 365 features:
- Outlook on the web, where users could have trouble opening mailboxes.
- The Outlook desktop app, which was affected by Exchange Online availability issues.
- Mobile email clients connected to Exchange Online, which could also face access problems.
- Teams calendar and meeting-scheduling functions, while Teams messaging and online meetings were largely still working.
- Copilot features that load conversations, generate responses or retrieve work information.
Microsoft said unavailable servers shifted load onto the remaining infrastructure. Some systems failed automated health checks, while the company also identified an overloaded process involved in establishing secure connections. Engineers paused deployments and configuration changes, restarted servers and rerouted traffic, but some machines reportedly stopped responding again after receiving traffic.
Microsoft said it had found no indication that malicious activity caused the outage. At the peak, more than 130,000 Outlook problem reports were logged in the United States on Downdetector. The disruption also affected public-sector organizations in several countries, along with Microsoft 365 connections used by OpenAI and Anthropic services.
In updates published on the morning of November 13, Microsoft said it was adding capacity through virtual machines and repairing blocked systems. Administrators can monitor the company’s Microsoft 365 service-health items EX1550543 and MO1550821; full service stability remained unconfirmed in the latest update.
