Microsoft is investigating an ongoing Outlook.com outage that is causing intermittent signing issues and preventing customers from accessing their mailboxes.

Since the incident started over three hours ago, outage monitoring service Downdetector has received thousands of user reports, with most affected users reporting login problems and connection issues.

In an update on the company's official Service Health Status page, Microsoft said that affected customers are being signed out of their accounts and are seeing "too many requests' errors.

"Some users may experience intermittent sign‑in failures, including 'too many requests' errors or unexpected sign‑outs," Microsoft said.

"Our investigation indicates client sign-in scenarios may be contributing to the reported behaviour, and we're focused on validating interactions across service components to identify steps."

Microsoft has yet to share more information about the outage's root cause and hasn't disclosed how many users or which regions were affected.

However, this incident has been flagged as causing "service degradation," a label typically used for issues with noticeable user impact, but that don't take the service offline for everyone.

Last month, Microsoft also mitigated an Exchange Online outage that blocked users from accessing mailboxes and calendars via Outlook on the web, Outlook desktop, Exchange ActiveSync, and other Exchange Online connection protocols.

The same day, it addressed a separate issue that caused Office.com or Microsoft 365 Copilot web sign‑in problems impacting the Microsoft Copilot desktop app, Copilot in Microsoft Teams, and Copilot in Office apps.

Update April 27, 09:46 EDT: Microsoft is blaming the ongoing Outlook.com sign-in issues on a "recently introduced change."

"We're reverting a recently introduced change to determine if this action provides relief from impact upon completion," the company said in a new Service Health Status update. "In parallel, we're continuing to analyze customer reports, and we're closely monitoring service telemetry to identify our steps."