Microsoft are enabling a feature as per Microsoft 365 Roadmap ID 88381, which will allow Teams chat with external users using personal Microsoft accounts (e.g. Hotmail, Outlook etc). This potentially opens Teams to data loss or spam from external users, therefore many organisations will want to disable this feature. This is quick blog post which shows how to check and disable this.

Note that this setting will default to On unless you explicitly turn it off.

Here’s the original message in the message centre:

There are 2 ways to check and change this setting, using Teams Admin, and PowerShell.

The message also says how to turn it off:

Teams admin

For those who like the UI, this is easy to do:

• Logon to Teams admin at https://admin.teams.microsoft.com/
• Click Users > External access
• Under ‘Teams accounts not managed by an organization’ toggle the setting ‘People in my organization can communicate with Teams users whose accounts aren’t managed by an organization’ to off.

PowerShell

It’s also easy to achieve in PowerShell, and always nice to verify the setting is as you expect:

import-module microsoftteams
connect-microsoftteams
get-CsTenantFederationConfiguration # check the setting
set-CsTenantFederationConfiguration -AllowTeamsConsumer $false # turn it off

And that’s it! Super quick and easy and I’d recommend making this change for all new or existing tenants, unless you want Teams to be open to personal accounts.