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Starting on May 3, 2021, you can't start add-ins in an instance of Outlook on the web that's running on Microsoft Exchange Server 2016 RTM.
This change occurs because the Outlook add-ins use the Office JavaScript APIs that are accessed through the Office JS (Office.js) Content Delivery Network (CDN) to interact with content in Outlook on the web. These JavaScript APIs are deprecated for Exchange Server 2016 RTM.
After this change is made, you receive the following error message when you try to open an add-in in Outlook on the web:
This add-in has been disabled to help keep you safe.
To resolve this issue, upgrade the Exchange-based server to the latest cumulative update.
Training
Module
Build Office Add-ins for Outlook - Training
This module walks through development of Office Add-ins for Microsoft Outlook.
Certification
Microsoft Office Specialist: Outlook (Office 2016) - Certifications
Demonstrate that you have the skills needed to get the most out of Outlook 2016 by earning a Microsoft Office Specialist (MOS) certification.
Documentation
Troubleshoot Outlook contextual add-in activation - Office Add-ins
Possible reasons your contextual Outlook add-in doesn't activate as you expect.
Deploy and install Outlook add-ins for testing - Office Add-ins
Create a manifest file, deploy the add-in UI file to a web server, install the add-in in your mailbox, and then test the add-in.
Outlook add-in requirements - Office Add-ins
For Outlook add-ins to load and function properly, there are a number of requirements for both the servers and the clients.