Microsoft Exchange Online buckles: email delays, sign-in errors hit tens of thousands

Key points

• Microsoft opened an active investigation into an Exchange Online outage at 5:30 PM UTC, logged in its admin centre as incident EX1464935.
• Tens of thousands of users reported email delays, failed deliveries and sign-in errors on Downdetector.
• Microsoft says it has isolated a common failure pattern tied to authentication and protocol connectivity across affected requests.
• This is the third notable Exchange Online outage this year, after separate incidents in April and June 2025.

Microsoft is scrambling to fix a live outage in Exchange Online, the cloud version of its corporate email service used by a large share of the world's offices.

Staff at affected companies are seeing emails arrive late or not at all. Others cannot log in to Outlook or the Exchange admin tools at all.

The fault was first flagged by users on social media before Microsoft acknowledged it, and picked up by BleepingComputer as reports spread.

What is actually broken?

Microsoft says it has found a shared failure pattern involving authentication, which is the step where the service checks that you are who you say you are before letting you into your mailbox. That step is misfiring for many customers, which cascades into the other symptoms.

In its admin centre update, the company listed four visible problems:

• Delays or outright failures when sending or receiving mail.
• Sign-in errors when trying to reach Exchange Online.
• Trouble opening or using the Exchange administration tools.
• Intermittent failures on mailbox actions and message delivery.

Microsoft has classified the event as an incident, the label it reserves for critical service problems with clear user impact. It has not yet said which regions are worst hit.

"We've isolated a common failure pattern across affected Exchange Online requests that is associated with authentication and protocol connectivity," the company wrote. Engineers are combing through telemetry, the internal performance data Microsoft's services report back, to pin down the root cause.

Who is affected?

Any organisation that runs its email through Microsoft 365 could be caught up in this. That covers a huge slice of hospitals, councils, schools, law firms and small businesses.

Outage tracker Downdetector logged tens of thousands of user reports, with complaints centred on Outlook not receiving messages and Exchange Online refusing logins.

If you are a customer or patient of an affected organisation, expect slower email replies today. A booking confirmation or appointment reminder that normally lands in seconds may take hours, or bounce back to the sender and need re-sending later.

Is this a security breach?

No, based on what Microsoft has said so far, this looks like a service fault, not an attack. Authentication systems can break for entirely mundane reasons: a bad configuration push, a certificate that expired, a token service that fell over under load.

That said, an authentication outage is a useful moment for scammers. When people cannot log in normally, they are more likely to click a plausible-looking "reset your Microsoft password" email without thinking. Treat any such message today with suspicion and go to the login page directly instead.

A rough year for Exchange Online

This is the third headline outage for the service in 2025.

Microsoft resolved each of the earlier incidents, but the pattern is uncomfortable for a service that businesses treat as plumbing. When Exchange Online sneezes, a lot of the working world catches it.

This is a developing story and we will update it as Microsoft shares more detail on the cause and the fix.