More than 1,500 users were locked out of Outlook starting around 4 a.m. ET Monday, April 27, as sign-in failures and authentication errors swept across the service. The outage stretched past eight hours before Microsoft resolved it through traffic rebalancing rather than the configuration rollback it tried first.
Downdetector logged peak reports of 1,516 by mid-morning, with complaints holding steady at roughly 1,400 per hour through the afternoon. Users reported intermittent sign-in failures, "too many requests" errors, and unexpected sign-outs on both browser and mobile versions of Outlook.
Microsoft acknowledged the issue at 6:20 a.m. ET via its Microsoft 365 X account, saying it was investigating problems accessing Outlook.com. The company quickly identified "an unexpected increase in error rates affecting two separate error scenarios" and began rolling back a recent backend configuration change.
That rollback did not work. "The roll back that was completed does not appear to have provided the intended impact relief," Microsoft wrote in a status update hours later. The company pivoted to a different theory. An internal investigation revealed that a portion of Microsoft's Copilot infrastructure in North America was consuming excessive resources, forcing engineers to rebalance traffic across the affected systems.
After reallocating capacity and optimizing infrastructure, Microsoft reported positive signals in service health around midday.
IOS users faced additional hurdles even after the broader fix took hold. Microsoft published a manual workaround requiring iPhone users to handle through Settings > Mail > Accounts and re-enter their passwords before Outlook would sync again on Apple devices. The final resolution came when "actions to rebalance traffic and optimize infrastructure" fully restored service, according to the company's status page. Outage reports on Downdetector dropped sharply by late afternoon.
Monday's disruption only affected consumer Outlook accounts accessed through Outlook.com or personal-use subscriptions. Business users on Microsoft 365 commercial plans were not impacted.
