Microsoft Outlook outage cause seemingly revealed

Updated on Friday, Nov. 13 at 2:11 p.m. ET — Microsoft has reported "broad recovery and stability" across its services and published an apology on X. We've updated this story to reflect the latest updates from the company.

Updated on Friday, Nov. 13 at 1:08 a.m. ET — Microsoft is still looking into the cause of the outage, but states that at present it is "not aware of any malicious activity that is causing this incident."

Updated on Thursday, Nov. 12 at 10:58 p.m. ET — Unfortunately, Microsoft's efforts to bring servers back online aren't going as planned. "We've monitored the status of the changes applied to improve serviceability and confirmed they're not taking as expected," Microsoft wrote in an update on X.

Updated on Thursday, Nov. 12 at 8:07 p.m. ET — Microsoft has revised its list of features and services impacted by this outage to include Microsoft Outlook, SharePoint Online, Teams, Copilot AI, and Microsoft 365, though Azure appears to be operational.

Updated on Thursday, Nov. 12 at 7:53 p.m. ET — Microsoft has issued a new update stating that the servers they've brought back online may become unresponsive again when they receive traffic. The company is currently investigating the issue and limiting the types of traffic going to these servers in some locations.

Updated on Thursday, Nov. 12 at 6:23 p.m. ET — Microsoft states that it is bringing some servers back online, and "they remain stable so far." Users should expect a "gradual recovery."

Updated on Thursday, Nov. 12 at 5:26 p.m. ET — Microsoft services remained down on Thursday evening, as Microsoft acknowledged ongoing problems in an update posted to X.

If Outlook wouldn't load for you on Thursday (or on Friday morning), Microsoft now has a theory about what went wrong.

In its initial update about the outage, Microsoft stated that Microsoft Exchange Online was at the root of the problem. Many users were unable to access Microsoft 365 services related to Exchange Online, including Outlook, Teams, SharePoint Online, Copilot, Defender XDR, and more. Exchange Online is the back-end, cloud-based tool that provides data updates for front-end, user-facing tools like Microsoft Outlook.

In updates posted to X and on its Service Health Status Page, Microsoft stated that some of its servers became "unexpectedly unavailable" on Thursday. With parts of its infrastructure under a heavier-than-usual load across multiple regions, the company said the extra strain was likely contributing to connection problems.

The company also stated in an X post that the failures may have been caused by a "specific component that's responsible for processing server health," and that servers were going offline after failing automated health checks.

When Microsoft found that the initial fixes weren't having the desired effects, the company also tried rolling back recent changes to its servers, while continuing to balance traffic across its network. The company is also investigating why the health checks failed in the first place.

At first, Microsoft stated that it hadn't found a recent change that coincided with the start of the outage.

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However, in a Friday morning statement posted to X, the company attributed the outage to a recent network update. "The incident was introduced by an issue during an earlier network capacity upgrade that surfaced following a recent network change. We have addressed the underlying issue and continue to restore service to remaining customers and regions."

While Microsoft has now provided a partial explanation of what caused the global Microsoft outage, the picture remains incomplete.

Microsoft pledged to provide a "full post-incident report" detailing what went wrong. The company also issued a formal apology to its customers.

"We are deeply sorry for the disruption this incident has caused our customers and partners," the company wrote on X. "We know the trust they place in our services and take that responsibility extremely seriously. We are seeing broad recovery and stability across our services."

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Microsoft rules out a cyberattack or rogue AI agents as cause of Outlook outage

As the widespread Microsoft outage continued into a second day, Microsoft issued several updates addressing rumors that a cyberattack or AI agents were responsible for the outage.

In an update posted on its Service Health Status page, the company wrote, "This was a technical issue not related to cyber or AI / agents."

In an earlier statement, the company wrote that it was "not aware of any malicious activity that is causing this incident."

A brief timeline of the Microsoft outage

On Thursday, a Microsoft outage affected Outlook, Teams, and other Microsoft 365 services, starting around 12:30 p.m. ET.

By 1 p.m. ET, nearly 130,000 user error reports were submitted to Downdetector, and users reported trouble loading their inboxes and sending or receiving emails. Copilot users also reported problems.

Microsoft initially flagged trouble accessing Exchange Online mailboxes in the Americas. Exchange Online is the service that stores and delivers email for many Outlook users.

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The company later warned that the issue "may affect Microsoft 365 service connectivity for users in any region."

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The outage dragged out into the evening, and eventually Friday morning, before Microsoft's mitigations began to take effect. By 7:09 a.m. ET, the company announced on X that it was finally seeing gradual recovery.

"We’re seeing encouraging recovery progress, with service availability and email delivery performance improving across a growing portion of affected Microsoft 365 infrastructure. Some users may still experience intermittent issues as recovery actions continue."

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In response to Mashable's questions about the outage, Charles Lamanna, EVP, Copilot, Agents and Platform at Microsoft, provided the following statement:

 "We’re seeing encouraging recovery progress, with service availability and email delivery performance improving across a growing portion of affected infrastructure. We know our customers rely on our services to get important work done every day, and regret the impact these outages had on those who depend on us. Our teams are reviewing what happened and how we can prevent similar issues going forward."

This is a developing story...

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