Microsoft Teams: New tenant federation setting to block all subdomains of domains in blocklist

If your organization is using a Block list to protect your users from malicious or other domains on Microsoft Teams, there is a new setting available which enables you to block all subdomains of domains in the federation Block list. This setting can be enabled using a PowerShell cmdlet and affects new or existing chats with users from blocked subdomains. By enabling this new setting, your organization will also be protected from all related subdomains automatically without manually adding all subdomains to the Block list.

It is essential to note that the new -BlockAllSubdomains parameter in the PowerShell cmdlet is disabled by default, hence there won't be any impact on your organization if administrators take no action. However, to enable this setting, use this PowerShell cmdlet: Set-CsTenantFederationConfiguration -BlockAllSubdomains $True

With this new block list integration, most organizations should consider enabling this new setting to protect their users from all related subdomains without manually adding each subdomain to the Block list. Existing 1:1 chats with users from blocked subdomains will be disabled, and in the existing chat groups with users from blocked subdomains, users from the blocked subdomains will be removed. This setting does not affect organizations using closed federation and who have configured the list of allowed domains. Rollout for this feature started mid-March 2024 and was completed in early April 2024.

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