@StephanGee  Hi it is possible to create a controlled list of domains for Federated access from the Teams Admin Centre

The policy is under Org-Wide Polices the select the External Access policy

 

There is the option to either create an Allowed or Blocked list, but not both

 

See Microsoft Doc article for more infor. https://docs.microsoft.com/en-us/microsoftteams/manage-external-access 

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