Microsoft Outlook is experiencing a widespread outage Monday August 31, the last day of the summer, that started around 11:30 AM Eastern and had generated nearly 50,000 user reports on Downdetector by early afternoon.
Most users reporting issues say they cannot receive messages. Microsoft's own status page initially listed all products as operational before the company acknowledged the problem.
The culprit: an authentication component that is preventing parts of Microsoft's infrastructure from processing email traffic as expected.
"We've confirmed that users may experience degraded functionality with various Exchange Online functionalities," Microsoft 365's official status account posted.
"We've identified an issue with an authentication component which is contributing to impact. To address this, we've developed a remediation strategy and we're applying it to a portion of infrastructure to test its efficacy."
The authentication issue is not limited to Outlook, Microsoft confirmed additional services beyond Exchange Online are also affected and directed administrators to check the admin center for case number EX1464935.
Office workers returning from the long Labor Day weekend found their inboxes frozen at the worst possible time, the first business morning of September.
Microsoft has not said what caused the misconfiguration or when a full fix will be deployed. The company is testing its remediation on a portion of infrastructure before rolling it out more broadly.