Microsoft is currently investigating a significant bug affecting classic Outlook for Windows that prevents users from accessing their email accounts.
The issue manifests as a persistent error message stating “Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened. The attempt to log on to Microsoft Exchange has failed.”
The error appears when users attempt to launch the traditional Outlook desktop application on Windows devices.
According to Microsoft’s official documentation updated on September 26, 2025, this particular error can occur for various reasons, but recent support cases have primarily involved user mailboxes experiencing authentication failures.
Microsoft has identified specific technical indicators to confirm whether users are experiencing this particular issue.
Technical diagnostics reveal the error stems from a “ClientBackoffException” where the authentication concurrency limit is reached, causing the Exchange server to reject connection attempts.
This bug significantly impacts business productivity as affected users cannot access their email through the traditional Outlook desktop client.
Organizations relying on classic Outlook for daily communications face disruptions to their normal workflows, potentially affecting meeting scheduling, document sharing, and general correspondence.
The timing of this issue is particularly concerning for businesses as it affects one of the most widely used email clients in corporate environments.
Users experiencing this problem find themselves unable to access stored emails, calendar appointments, or contacts through their preferred desktop application.
Microsoft has classified this issue as “under investigation” and currently offers limited resolution options.
The company acknowledges that the only definitive way to address the problem requires opening a support case through the Microsoft 365 Admin portal, where Exchange Online support can request service-level changes to mitigate the issue.
This approach indicates the problem likely stems from server-side configurations or limitations rather than local client issues, explaining why standard troubleshooting methods prove ineffective.
While Microsoft investigates the root cause, affected users have two primary alternatives to access their email accounts.
The first option involves using Outlook Web Access (OWA), which allows full email functionality through a web browser without requiring the desktop application.
Outlook Web App interface showing an email inbox with a message open and permissions menu visible
Additionally, users can switch to the new Outlook for Windows, Microsoft’s modernized email client that operates independently of the classic version.
This newer application provides similar functionality while avoiding the authentication issues plaguing the traditional client.
Organizations experiencing this issue should consider temporarily migrating affected users to the recommended workarounds while awaiting a permanent fix.
Microsoft continues investigating the underlying cause and expects to provide updates through official support channels as the situation develops.
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