Microsoft (Nasdaq: MSFT) returned some servers to service Thursday after a failure in Exchange Online cut off Outlook and other Microsoft 365 tools for hours, but it had not declared the outage over or named a cause.
The outage began around 11:17 a.m. ET, when users in the Americas lost access to Exchange Online mailboxes. Microsoft later said users in any region could be affected, and it listed Teams, SharePoint, Copilot and Defender XDR among the services hit. Outlook outage reports on Downdetector topped 130,000 in the early afternoon and still stood near 31,000 at 8 p.m. ET.
Microsoft first blamed elevated load, then said traffic had not increased, and servers were unexpectedly unavailable, straining the rest of its infrastructure. It froze all deployments and configuration changes to check whether a recent change caused the problem, and by 2:43 p.m. ET it said affected servers were failing automated health checks.
Around 4 p.m. ET, Microsoft said a replacement health check brought some affected servers back online and began applying it more broadly. It warned that restored servers might stop responding again once traffic returned, and engineers are considering undoing a handful of recent setting changes to speed recovery.
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Microsoft has yet to explain what went wrong and has not suggested a cyberattack.
