From January 2026, you’ll be able to block external Teams users directly from the Defender portal using the Tenant Allow/Block List.
What stands out to me isn’t the ability to block external users. Teams could already do that in fragments. The real change is ownership. Blocking external communication is now enforced from the same place where email and other collaboration risks are already managed.
- External chats, meetings, calls, and channel messages can now be blocked centrally.
- Block by domain (up to 4,000) or specific email addresses (up to 200).
- This applies to all Teams clients and the Defender XDR web portal.
- Existing conversations from those users are removed automatically.
- No impact on your current Teams federation or domain settings.
👉🏻This works only if the Teams admin center toggles are enabled (they’re OFF by default).
You can find this news posted here: https://admin.cloud.microsoft/?#/MessageCenter/:/messages/MC1200058