Microsoft Investigates Major Outlook Bug Disrupting Mail Access
Microsoft is currently investigating a significant bug in the classic Outlook for Windows client. The issue, which has been acknowledged by the company, prevents users from accessing their mailboxes and displays a critical error message, disrupting daily workflows. The problem occurs when a user attempts to open the classic version of Outlook for Windows.
Microsoft has confirmed that the bug is due to the user's client exceeding the authentication concurrency limit set by the server. The issue appears to be linked to Microsoft 365 logon attempts. Organizations are advised to communicate these workarounds to their users to minimize disruption. Microsoft has provided immediate workarounds, including using Outlook Web Access (OWA) or switching to the new Outlook for Windows client.
As of September 26, 2025, Microsoft's official status for the issue is 'INVESTIGATING' and there is no direct fix available. Organizations must open a support case through the Microsoft 365 Admin portal to resolve the problem. Microsoft has also provided a diagnostic method for IT administrators to confirm if their users are impacted by the bug.
Microsoft is working on fixes for the bug in classic Outlook for Windows, which causes the application to fail upon launch. The responsibility for the error lies with Microsoft, as the company has acknowledged the issue and is actively investigating it. Users and organizations are advised to use the provided workarounds to mitigate the disruption caused by the bug.
