The message "Update to keep using Outlook" typically indicates that the version of the Outlook client being used is outdated and requires an update to continue functioning properly. This can occur if the user has not installed the latest version of Outlook from the Microsoft Store or if there are compatibility issues with the current version and the Microsoft 365 services.
For organizations using Microsoft 365 apps for Enterprise, it's important to ensure that users have access to the latest updates. If the Microsoft Store is blocked in your organization, users may need to install the new Outlook using an alternative method, such as downloading the installer directly from the Office CDN.
You can find more information on resolving installation issues and ensuring users have the latest version of Outlook in the following resources:
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