Microsoft has confirmed it is investigating a significant bug in the classic Outlook for Windows desktop client that causes the application to fail upon launch.
The issue, which appears to be linked to Microsoft Exchange logon attempts, prevents users from accessing their mailboxes and displays a critical error message, disrupting daily workflows for affected individuals and organizations.
The problem manifests when a user attempts to open the classic version of Outlook for Windows. Instead of loading the mailbox, the application presents an 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.”
While this generic error can stem from various causes, Microsoft has noted that recent support cases point to a specific issue with user mailboxes attempting to connect to the service.
Technical Details of the Outlook Bug
For IT administrators and technical teams looking to confirm if their users are impacted by this specific bug, Microsoft has provided a clear diagnostic method. By capturing a Fiddler trace while reproducing the error, administrators can search for a specific exception.
The presence of the following error in the trace log confirms the issue:
Microsoft.Exchange.RpcClientAccess.ServerTooBusyException: Client is being backed off —> Microsoft.Exchange.RpcClientAccess.ClientBackoffException: ErrorCode: ClientBackoff, LID: 49586 – Authentication concurrency limit is reached.
This exception indicates that the connection is failing because the user’s client is exceeding the authentication concurrency limit set by the server, effectively being “backed off” or throttled by Microsoft Exchange Online.
This prevents the Outlook client from successfully logging in and opening the user’s mailbox folders.
As of September 26, 2025, Microsoft’s official status for the issue is “INVESTIGATING.” There is currently no direct fix that users or administrators can apply themselves.
To resolve the problem, organizations must open a support case through the Microsoft 365 Admin portal. The Exchange Online support team will then need to implement a change on the backend to mitigate the authentication issue for the affected mailboxes.
In the meantime, Microsoft has provided immediate workarounds to ensure users can continue to access their email. Individuals affected by this issue are advised to use Outlook Web Access (OWA), the browser-based version of Outlook, or switch to the new Outlook for Windows client.
These clients are not affected by the bug and provide a reliable alternative while a permanent solution for the classic desktop application is being developed. Organizations are encouraged to communicate these workarounds to their users to minimize disruption.
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Source: cybersecuritynews.com
