Microsoft is further expanding the security capabilities of Teams with the option to block external Teams messages.
Starting in early January, security administrators can block external Teams users, so they can no longer send messages, calls, or meeting invitations to employees within the organization. The new feature is integrated into Microsoft Defender for Office 365 and allows blocked external contacts to be managed via Microsoft Defender.
Two settings required
To use the feature, administrators must first activate two standard disabled options in the Teams admin center:
- Block specific users from contacting people in my organization
- Allow my security team to manage blocked domains and blocked users
After that, security administrators can add, remove, and view external users and domains from Defender. The system supports up to 4,000 blocked domains and 200 email addresses.
Targeted at abuse and social engineering
According to Microsoft, the feature is intended to counter abuse of Teams by cybercriminals. They are increasingly using the platform for social engineering attacks targeting employees. In addition to this feature, Teams will also warn administrators of suspicious activity from external domains. In January, additional security measures will be enabled by default, such as the detection of malicious URLs and protection against dangerous file types.
The rollout will start in early January 2026 and should be completed by mid-January. The feature is available for organizations that use Teams in combination with Microsoft Defender for Office 365.