Microsoft's diagnosis: a missing MAPI property after a recent build
Microsoft says it has identified a repeatable trigger for the disappearance of Copilot and Copilot Chat buttons in Classic Outlook for Windows: the behavior appears after upgrading Classic Outlook for Windows to build 20026.20182 and higher. Current analysis, the company wrote, indicates that "in affected environments, Outlook is unable to locate the MAPI property PR_PROFILE_USER_SMTP_EMAIL_ADDRESS_W within the null profile section." As a result, Microsoft says, "the Outlook Copilot component cannot persist in Copilot-related settings, which may prevent the Copilot entry point from appearing in the Outlook navigation pane."
Who is affected and how the failure shows up
Microsoft clarified that the problem affects only customers licensed for Copilot Chat (Basic) or for paid Microsoft 365 Copilot (Premium). Affected users may see one or more of the following symptoms in Classic Outlook:
- The Copilot button is missing from the top-right area above the ribbon.
- The Copilot icon is missing from the left app bar or the More Apps area.
- In Add Apps, Copilot may appear as an available app, but selecting Open does nothing.
- Adding Copilot through ribbon customization may show the command as unavailable or grayed out.
- Copilot nevertheless remains accessible from other entry points, for example Outlook on the web or the Microsoft 365 Copilot standalone app or web experience.
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See what we buildWorkarounds Microsoft recommends
While the Outlook team continues work on a permanent fix, Microsoft published a temporary workaround that restores Copilot entry points in Classic Outlook for some users. The recommended change is to enable the "Show Apps in Outlook" option in Classic Outlook settings: select the File tab, then Options, go to Advanced, and check the box for "Show Apps in Outlook" under "Outlook panes." Microsoft also notes that affected users can create a new Outlook profile or switch to the new Outlook email client or to Outlook Web Access (OWA), which the company says are not affected by this bug.
Kaspersky interaction: Outlook crashes and Event 1000
In a separate confirmation, Microsoft acknowledged an additional bug that can trigger unexpected Outlook crashes on systems running Kaspersky Antivirus. The company tied the crashes to "Event 1000" Application log entries and to the Kaspersky Mail Checker component (mcou.dll). The Outlook team is investigating the issue and advised Kaspersky customers who encounter crashes to contact Kaspersky support for assistance.
What this means for technologists, procurement leads, and end users
- Technologists and security teams: track deployments to Classic Outlook build 20026.20182 and higher, and validate whether affected endpoints have lost Copilot UI elements; enable "Show Apps in Outlook" where appropriate and consider profile recreation or migration to the new Outlook client or OWA as interim mitigations.
- Enterprises and procurement leaders: note that the impact is limited to accounts with Copilot Chat (Basic) or paid M365 Copilot (Premium) licenses; communicate to end users how to reach Copilot via Outlook on the web or the standalone Copilot experience while a Microsoft fix is pending.
- End users: if Copilot buttons vanish, try the "Show Apps in Outlook" option in File → Options → Advanced, create a new Outlook profile, or switch to Outlook Web Access or the Microsoft 365 Copilot web/app entry points until the Outlook Team issues a permanent remedy.
Microsoft had previously announced fixes earlier in the year for separate reliability problems — including issues that made the Classic Outlook client unusable when enabling the Microsoft Teams Meeting Add-in and incidents that prevented some Classic Outlook users from sending mail via Outlook.com — but for the Copilot disappearance the company says it is still investigating and working toward a permanent repair.
