Microsoft probes authentication glitch behind multi-hour Outlook outage

On Monday, Microsoft acknowledged a widespread outage affecting its Exchange Online service, which powers Outlook for business customers. The disruption, lasting several hours, led to delayed or failed email delivery, authentication problems and impaired mailbox functions such as search and task handling. The company identified the root cause as a faulty authentication component and began testing a remediation on a segment of its network, planning to expand the fix if successful.

Outage reports started surfacing around 11:30 AM ET, and by early afternoon more than 5,000 users had reported issues on Downdetector, with additional complaints on X. Affected users saw errors when sending or receiving messages, logging in, and performing other Exchange-related tasks. Microsoft said it was reviewing telemetry and diagnostics to fully isolate the issue.