The admin deploying add-ins and the users receiving those add-ins must be on a version of Exchange Server that supports OAuth authentication.

OAuth Exchange Authentication: Microsoft Exchange stores the add-in manifests within your organization’s tenant.

Under Product Information, you should see Microsoft 365 Apps for the enterprise. You can check this manually by going to the application, such as Word, then select “File” > “Account”.

For add-ins to be deployed the user must have Office ProPlus or Microsoft 365.

Outdated Office: The user might be on an older, incompatible version of Office.

Microsoft Teams Add-in disappears from the Outlook ribbon as follows: This makes the user frustrated while scheduling group discussions or meetings as they need to go back to old ways i.e., to start Microsoft Teams separately and schedule their meetings and work from there. In this case, even though users successfully install Microsoft Teams Add-in for Outlook (doesn’t matter automatically or manually), they still can’t access it in Microsoft Outlook. Lately, people have been reporting missing Microsoft Teams Add-in for Outlook. But these recent build-upgrades from Microsoft are not entirely error-free. Once you have installed the Microsoft Teams add-in for Outlook, a new icon will appear in the ribbon at the top of the Outlook page.

Users can quickly access Microsoft Teams directly from their Outlook email client through a simple click. Microsoft Teams-Outlook integration is one such example. Microsoft wants its users to be able to efficiently switch from one product to another.

Microsoft office icons missing