Microsoft Outlook Bug on Windows Devices Results in Repeated Email Crashes

Microsoft is examining a significant issue affecting the classic Outlook application for Windows. This bug prevents users from accessing their email accounts.

When attempting to start the Outlook desktop application on Windows devices, users encounter the following error message: "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."

As per Microsoft's official documentation updated on September 26, 2025, this error may occur due to various reasons, with recent support cases largely involving user mailboxes experiencing authentication failures.

The error is linked to a "ClientBackoffException," which occurs when the authentication concurrency limit is reached, resulting in the Exchange server rejecting connection attempts.

This issue significantly disrupts business productivity, as affected users are unable to access emails via the traditional Outlook desktop client. Organizations that rely on this application for daily communications may experience interruptions in meeting scheduling, document sharing, and overall correspondence.

Microsoft has designated this issue as "under investigation" and currently provides limited resolution options. The primary method to address the problem involves opening a support case through the Microsoft 365 Admin portal, permitting Exchange Online support to request service-level changes to mitigate the issue.

This indicates that the problem likely originates from server-side configurations or limitations, rather than local client issues, explaining the ineffectiveness of standard troubleshooting methods.

While the investigation continues, affected users have two primary alternatives for accessing their email accounts:

Using Outlook Web Access (OWA) for complete email functionality through a web browser. Switching to the new Outlook for Windows, a modernized email client that operates independently of the classic version, thus avoiding the authentication issues.

Organizations should consider temporarily migrating affected users to these workarounds while awaiting a permanent solution. Microsoft is actively investigating the root cause and will provide updates through official support channels as developments occur.

Based on reporting by GBHackers.