Microsoft says the Outlook Team has addressed this issue with a service change on June 29, 2026.
What broke in Classic Outlook for some Copilot users
Windows users holding the Copilot Chat (Basic) license experienced disappearing Copilot controls in Classic Outlook. A Microsoft support document lists the visible symptoms: the Copilot button vanishing from the top-right area above the ribbon; the Copilot icon missing from the left app bar or the More Apps area; and attempts to open Copilot from Add Apps returning no response. Ribbon customization could show the Copilot command as unavailable or grayed out. Microsoft noted that Copilot functionality remained available through other entry points such as Outlook on the web or the Microsoft 365 Copilot standalone app or web experience.
Fix deployed and recommended immediate steps
Microsoft applied a service-side change on June 29, 2026 to address the missing-button issue. The company advised affected users who still cannot see Copilot buttons in Classic Outlook to restart their email client to obtain the change immediately. In addition, customers are recommended to update to the latest build by selecting File > Office Account > Update Options > Update Now.
Workarounds when updating isn’t possible
For organizations or devices that cannot upgrade their Outlook client immediately, Microsoft documented two alternate paths. Administrators or users may revert to the previous Current Channel build (16.0.20026.20168) to restore prior behavior, or they may shift to the new Outlook or to Outlook Web Access (OWA) to continue using Copilot entry points without interruption.
Kaspersky-related Outlook crashes under investigation
Separately, Microsoft is investigating a known issue that can cause unexpected Outlook crashes on systems running Kaspersky Antivirus software. The crashes are linked to the Kaspersky Mail Checker module (mcou.dll). Outlook for Microsoft 365 users who suspect this crash are advised to check the Application log for "Event 1000" records showing OUTLOOK.EXE as the faulting app name and MCOU.DLL as the faulting module name. "If you are experiencing this crash, please contact Kaspersky support," the Outlook Team said.
What this means for enterprise admins, technologists, and end users
- Enterprise administrators: Restarting affected clients and pushing the Microsoft update are the fastest remedies; if upgrades are constrained, reverting to build 16.0.20026.20168 or directing users to the new Outlook or OWA are explicit mitigations documented by Microsoft.
- Technologists and security teams: Teams diagnosing crashes tied to third‑party antivirus should examine Application logs for Event 1000 records that identify MCOU.DLL as the faulting module, and coordinate with Kaspersky support if the crash pattern appears.
- End users with the Copilot Chat (Basic) license: If Copilot controls are absent from Classic Outlook, restart the client and run the Office update flow (File > Office Account > Update Options > Update Now). If problems persist, use Outlook on the web or the Microsoft 365 Copilot standalone app or web experience as noted by Microsoft.
Microsoft also noted that, in recent months, it resolved other known Classic Outlook problems — including issues that prevented some Classic Outlook users from sending emails via Outlook.com and problems that rendered the client unusable for users who enabled the Microsoft Teams Meeting Add-in. For the immediate Copilot-button outage, the company’s June 29 service change and the documented update and rollback paths give administrators and users concrete, actionable steps to restore functionality.
