A significant bug is currently preventing some users from accessing their email through the classic Outlook for Windows desktop application. When affected users attempt to launch the program, they are met with 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.” Microsoft’s official documentation, updated on September 26, 2025, confirms that while this error can have various causes, recent support cases have primarily involved user mailboxes experiencing authentication failures. Technical diagnostics reveal the issue stems from a “ClientBackoffException” where the authentication concurrency limit is reached, causing the Exchange server to reject connection attempts. This server-side problem explains why standard local troubleshooting methods are ineffective, as the core issue isn’t within the user’s client.
This bug is significantly impacting business productivity, as users cannot access their email, calendar appointments, or contacts through the traditional Outlook desktop client. Organizations that rely on classic Outlook for daily communications are facing disruptions to their normal workflows, which affects everything from meeting scheduling to general correspondence. The timing of this issue is particularly concerning because it affects one of the most widely used email clients in corporate environments, and many businesses depend on it for critical operations. The problem’s server-side nature further complicates matters, as it prevents users from independently resolving the issue and forces them to seek official support.
Microsoft has classified the issue as “under investigation” and offers limited resolution options at this time. The company acknowledges that the only definitive way to address the problem is to open a support case through the Microsoft 365 Admin portal. Once a case is opened, the Exchange Online support team can request specific service-level changes to mitigate the issue. This process confirms that the problem likely originates from server configurations or limitations rather than local client-side bugs. Affected users must rely on Microsoft’s support team for a permanent fix, which can be time-consuming and disruptive.
While Microsoft continues to investigate the root cause, affected users have two primary alternatives to regain access to their email accounts. The first option is to use Outlook Web Access (OWA), which provides full email functionality through a web browser without requiring the desktop application. The second option is to switch to the new Outlook for Windows, Microsoft’s modernized email client. This newer application operates independently of the classic version and avoids the authentication issues plaguing the traditional client. Both of these workarounds provide a viable way for users to continue their work while awaiting a permanent solution.
Organizations experiencing this issue should consider temporarily migrating affected users to one of the recommended workarounds to minimize business disruption. These alternatives allow users to maintain access to their email accounts, calendars, and contacts without relying on the affected desktop application. Microsoft is continuing its investigation into the underlying cause and is expected to provide updates through its official support channels as the situation develops.
Reference:
