How did a Microsoft email failure disrupt services worldwide?

Detailed summary

An email outage disrupted work at the US House of Representatives, the Philippine Supreme Court and two Indian airlines as Microsoft struggled to keep servers online.

A global Microsoft 365 outage has left users unable to open their Outlook inboxes and disrupted parts of Teams and Copilot, as server failures that hit during the US working day on Thursday continued into Friday morning across Asia and Australia.

Early in the outage, Microsoft warned on its Microsoft 365 Status account on X that the problem “may affect Microsoft 365 service connectivity for users in any region”.

In the US alone, Downdetector recorded more than 130,000 reports of Outlook problems at around 0100 PM Eastern time on Thursday.

Several servers unexpectedly went offline, Microsoft said on X, leaving fewer machines to handle the usual volume of traffic. As the remaining servers came under strain, users began having trouble connecting to Exchange Online, the service that manages work email, calendars and contacts.

“We found a core process on affected servers is becoming overwhelmed, causing capacity loss,” Microsoft said in a later post on X.

In a service update carried by ABC News, the company explained that the process establishes secure connections. When it became overloaded, the servers stopped responding to requests. Microsoft said it had seen no indication that malicious activity had caused the outage, according to AP.

Outlook relies on Exchange Online to open a mailbox. Teams and Copilot also use it for some features, so the same failure could disrupt one part of an app while leaving another available, according to Reuters, citing Microsoft’s service notice.

The elements that did not rely on Exchange Online remained functional.

Exchange Online is the company's cloud-based messaging platform that delivers email, calendar, contacts, and tasks.

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