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.” 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.