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 and sign-in failures on Downdetector.
• Microsoft has isolated a common failure pattern tied to authentication and protocol connectivity across affected requests.
• The service suffered separate notable outages in April and June 2025.

Microsoft is working to fix a live outage in Exchange Online, the cloud email service that runs inboxes for a large share of the world's offices.

Staff at affected companies are seeing emails arrive late or not at all. Others cannot get into Outlook or the Exchange admin tools at all. Users flagged the problem on social media before Microsoft acknowledged it, and BleepingComputer picked it up as reports spread.

What is actually broken?

Microsoft has found a shared failure pattern involving authentication, the step where the service confirms your identity before unlocking your mailbox. When that step misfires, everything downstream breaks with it.

The company listed four visible symptoms in its admin centre:

• Delays or outright failures sending or receiving mail.
• Sign-in errors when reaching Exchange Online.
• Trouble accessing 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, and has not yet named 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 in its admin centre. Engineers are combing through service telemetry, the internal performance data Microsoft's systems report back, to find the root cause.

Who is affected?

Any organisation running email through Microsoft 365 could be caught. That covers hospitals, councils, schools, law firms, small businesses and plenty in between.

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

If you are a customer or patient of an affected organisation, expect slower replies. A booking confirmation that normally lands in seconds may take hours, or bounce and need re-sending.

We reported a related stumble just last week, when a botched Microsoft deployment broke search across Outlook, SharePoint and OneDrive. This authentication fault is a separate incident, but the proximity is the kind of pattern worth watching.

Should you worry about a security breach?

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, an expired certificate, a token service that fell over under load. An outage is not evidence of a compromise.

Authentication outages do, though, create an opening for phishing. When people cannot log in normally, they are more likely to click a convincing-looking password-reset email without stopping to think. Go directly to the login page; do not follow links from unexpected messages.

A rough year for Exchange Online

Microsoft resolved each earlier incident, but the frequency is uncomfortable for a service 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.