Microsoft Confirms and Fixes Classic Outlook Crash Caused by Teams Meeting Add-in

Starting around March 12, 2026, classic Outlook for Windows began crashing on launch and prompting users to restart in Safe Mode. Microsoft confirmed the problem on March 17 and is tracking it as Microsoft 365 advisory EX1254044, categorized as a service degradation affecting Exchange Online users. The trigger is a compatibility conflict: older builds of classic Outlook running the newest Microsoft Teams Meeting Add-in (build 1.26.02603) crash when the add-in initializes. Disabling the add-in stops the crashes. Importantly, this is a client stability bug - not a security vulnerability - so it is documented through Microsoft's service-health and support channels, not the Security Response Center or any exploited-vulnerability catalog.

The crash affects classic Outlook for Windows on Current Channel builds equal to or lower than Version 2402 (Build 17328.20142) when the Teams Meeting Add-in is at build 1.26.02603. In other words, it is older Outlook builds that are vulnerable to the conflict - which is why updating Outlook resolves it. Microsoft confirms that new Outlook for Windows, Outlook on the web, Outlook mobile, and Outlook for Mac are not affected, because they do not load the classic COM add-in. Cached and online Exchange modes are both affected on impacted builds.

The conflict sits between the newer Teams Meeting Add-in and an older Visual C++ runtime shipped with affected Office builds. On impacted machines, the Windows Application event log records an Event 1000 crash with a faulting module of MSVCP140.dll and exception code 0xc0000005 (an access violation) as Outlook initializes the add-in. Because the fault occurs during startup, Outlook cannot open normally until the add-in is bypassed or the runtime is updated by moving to a newer Office build. The problematic Teams add-in arrived with a recent Teams client update, and the Teams Meeting button in classic Outlook is provided by a locally registered COM add-in (ProgID TeamsAddin.FastConnect).

Microsoft's primary guidance is to update classic Outlook to the latest build. Because Outlook itself is crashing, trigger the update from another Office app: open Word and go to File > Account > Update Options > Update Now, then reopen Outlook normally. If updating isn't immediately possible, an Online Repair of Office (which reinstalls Office components) is an alternative. As a temporary workaround to stay on an older build, disable the Teams Meeting Add-in: start Outlook in Safe Mode (hold Ctrl while launching, or run outlook.exe /safe), then go to File > Options > Add-ins, select COM Add-ins > Go, clear Microsoft Teams Meeting Add-in for Microsoft Office, and restart Outlook. Disabling the add-in removes the ability to add Teams links from Outlook's calendar until you update.

Microsoft developed a Teams-side fix included in Microsoft Teams version 26058.712.4527.9297, which began deploying on March 30, 2026. Combined with updating classic Outlook to a current build, this resolves the crash for affected users. Microsoft's guidance for anyone still hitting the issue is to ensure both classic Outlook and Teams are updated; the add-in also expects Outlook and Teams to run in the same architecture (both 32-bit or both 64-bit).

With both Outlook and Teams fixes rolled out, remaining exposure is limited to devices held back on older Office builds or delayed update rings. Admins on deferred channels (for example Semi-Annual Enterprise Channel) should confirm their builds include the fix before re-enabling the add-in fleet-wide. More broadly, the incident is a reminder that the classic Teams Meeting Add-in is a locally registered COM component tightly coupled to both Office and Teams servicing - a recurring source of Outlook add-in failures that the new Outlook for Windows avoids by not using it.