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TAC Gives External Collaboration Management a Makeover
In the article about developing user training for Microsoft 365 Copilot, I commented that keeping training updated to match changes made by Microsoft is a challenge. Teams was the first Microsoft 365 application to exhibit the kind of rapid development cycles that we currently see in Copilot. The Teams client UX seemed to change daily, and although the rate of change was slower for administrative functions and interfaces, it still happened.
Teams isn’t quite in the same place any longer. I guess that having hundreds of millions of users slows the ability of developers to make changes within an application. Major updates like the threaded layout for channels (after six months of trying, I still dislike the implementation) and minor changes like the arrival of autocorrect for message composition still happen, but at a more measured pace.
Open, Controlled, and Custom Modes for External Collaboration
All of which brings us to message center notification MC1183006 (last updated 1 December 2025) covering the introduction of three modes for external collaboration settings in the Teams admin center (TAC – Figure 1). The modes cover aspects of external collaboration such as shared channels, guest access to teams, and which external domains (including test domains) are allowed to chat with people in your tenant.
The new interface rolls out in general availability in mid-February 2026 and should be in place worldwide by the end of February 2026.
Working with Settings
Two of the modes (Open and Controlled) are predefined and the other (Custom) is what you’ll see if administrators have made any changes to the external collaboration settings in the past.
The idea of the two predefined modes is that tenants can choose a mode that makes sense for their operations. In truth, I don’t think that many tenants will end up in the open or controlled modes. Think of these modes as starting points to guide tenants to the most appropriate set of custom external collaboration settings.
Clicking the down arrow reveals details of individual settings, all of which have been available previously (details of the settings in the two predefined modes are available in the online documentation). For example, I have long advocated that Microsoft 365 tenants should use an allow list to control the tenants that users can collaborate with and the block list for the Entra B2B Collaboration policy to stop guests from being invited from specific domains. The allow list for tenants is available under the “Who gets access” settings.
However, the Entra B2B Collaboration policy settings aren’t currently shown in TAC. This isn’t surprising because the Entra policy affects all applications and TAC takes care of Teams-specific controls. It’s a good reminder that controls that affect Teams operations can exist elsewhere in Microsoft 365.
Making Teams Management More Elegant
It’s been said in the past that Teams is in danger of running up its real end due to the number of policies and settings available to manage different aspects of the application. There are too many policies in Teams and some of the policies are burdened by settings that go back to Skype for Business Online. If you’ve worked with Teams for a while, you might not notice just how many controls are available for tweaking, but it’s certainly an evident issue for people coming to Teams for the first time.
This isn’t a UX that most administrators are likely to use often. Once configured, external collaboration settings tend to stay in place until some reason arises to force an update, like when Microsoft blocked federated chat with trial tenants in 2024. Even so, it’s good to try and simplify tenant management, especially for a high-profile application like Teams, so this is a good change.
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