Microsoft is working to address a known issue that renders the classic Outlook email client unusable for users who have enabled the Microsoft Teams Meeting Add-in.

"Some users may be unable to use Microsoft Outlook Classic while the Microsoft Teams Meeting Add-in is enabled," Microsoft said in an admin center update under EX1254044.

While it didn't share more details about the root cause, Microsoft says the issue is due to a previous Outlook build.

To temporarily fix this bug, affected users are advised to update Outlook or to perform an Online Repair for click-to-run installs (which will reinstall all Office applications).

"Users attempting to use Microsoft Outlook Classic while the Microsoft Teams Meeting Add-in and previous Outlook build version is enabled may be impacted," Microsoft added.

"We've identified that a previous Outlook build version is causing impact to occur. We're working with your representatives to ensure that the latest Outlook version is enabled, to mitigate impact."

Microsoft is also investigating several issues that cause email connection problems in the classic Outlook desktop client and trigger 0x800CCC0F and 0x80070057 errors during synchronization with Gmail and Yahoo accounts.

In January, it addressed another classic Outlook issue caused by the December 2025 updates that prevented Microsoft 365 customers from opening encrypted emails.

On Monday, Redmond also mitigated a widespread Exchange Online outage (EX1253275) that prevented customers from accessing their mailboxes and calendars via Outlook on the web, Outlook desktop, Exchange ActiveSync, and other Exchange Online connection protocols.

This is a developing story...