Error when opening classic Outlook: "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."

Last Updated: September 26, 2025

ISSUE

Currently, when you open classic Outlook for Windows, you may get an 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."

This error message can occur for different reasons. Not every instance of this error is the same issue but recent support cases around this have been for user mailboxes.

To confirm if this issue applies, capture a Fiddler trace when you reproduce the problem and then search the trace for this error:

Microsoft.Exchange.RpcClientAccess.ServerTooBusyException: Client is being backed off
---> Microsoft.Exchange.RpcClientAccess.ClientBackoffException: ErrorCode: ClientBackoff, LID: 49586 - Authentication concurrency limit is reached.
--- End of inner exception stack trace ---

STATUS: INVESTIGATING

Currently, the only way to address this issue is to open a support case from the Microsoft 365 Admin portal. Exchange Online support will need to request a change from the service to mitigate.

WORKAROUND

To workaround the issue, use Outlook Web Access (OWA) or new Outlook for Windows.

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See Also

Fixes or workarounds for recent issues in classic Outlook for Windows