Microsoft has fixed a major bug preventing Microsoft 365 users from launching the classic Outlook email client on Windows systems.

As explained when it acknowledged the issue in September, affected customers are seeing errors saying that the app cannot be started, that the Outlook window cannot be opened, and that an attempt to log into the Exchange account 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," Microsoft said.

While it initially advised impacted users to open a support case with the Exchange Online support team to request a service change to mitigate it, Microsoft updated the support document on Monday to tag this known issue as fixed and added that the Outlook team is still monitoring to confirm it was fully mitigated.

"The service team implemented changes to address the issue. We are monitoring to confirm the issue is addressed," the company said.

​Microsoft also advised users who still can't open classic Outlook to switch to Outlook Web Access (OWA) or the new Outlook for Windows as a temporary workaround to access their mailboxes.

In a separate support document regarding "Cannot start Microsoft Outlook. Cannot open the Outlook Window" errors, Microsoft prompts users to follow these troubleshooting steps to resolve Outlook startup issues:

1. Start Outlook in safe mode and disable add-ins
2. Create a new Outlook profile
3. Repair your Outlook data files
4. Run the /resetnavpane command

Earlier this year, Microsoft also addressed classic Outlook bugs that broke email and calendar drag-and-drop functionality after installing Windows 24H2 updates and caused CPU spikes when typing messages.

Before this, Redmond fixed a bug that triggered classic Outlook crashes when starting new messages or opening emails and shared a temporary workaround for a known issue that caused Outlook errors when opening encrypted emails.