Unable to Communicate with Any Users in Microsoft Teams

Dear @Aravinth M,

Good day, and I appreciate the clear explanation of your concern. As a forum moderator, I genuinely wish I could directly access your account or delve into the backend systems to diagnose and fix this for you. However, our role here is limited to providing general guidance and solutions that can be applied by users. From my research, this is a very common Microsoft Teams issue. When your account appears as “Unknown User” to others (or to yourself in chats), messages often fail to deliver because Teams can’t properly resolve your identity. Please try the steps below to see if they help:

1.Sign out and sign back in (Most effective first step)

• Click your profile picture (top right) > Sign out.
• Completely close Teams (right-click the Teams icon in the system tray/taskbar > Quit).
• Re-open Teams and sign in again with your work/school account (the one from your company, not a personal @outlook.com or @hotmail.com account).
• Do this on both sides (you and the people you’re trying to message).

2.Clear Teams cache

• Type settings in the search box, and then select the Settings app from the results.
• Select Apps > Installed apps, and then type Microsoft Teams in the search box.
• Locate the New Microsoft Teams app from the results, select the More options button (...) on the right, and then select Advanced options.
• In the Reset section, select Reset.
• Restart Teams.

For reference: Clear the Teams client cache - Microsoft Teams | Microsoft Learn

3.Test in Teams on the Web

Go to teams.microsoft.com in your browser and sign in.

• If messaging works on the web but not in the desktop app > it’s definitely a cache issue (go back to step 2).
• If it still fails on the web > the problem is on the account/tenant side, please contact your IT administrator for further assistance.

I hope this information helps clarify the situation and provides you with workable solutions. Should you have any further questions or need additional assistance, please don't hesitate to reach out. I'm always happy to help. Have a wonderful day! 

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The behavior described matches two known scenarios in Microsoft Teams:

1. Account shows as “Unknown User”
   • If the account was deleted in the organization and then recreated, any historical references to the old account will permanently show as Unknown User.
   • If the account was disabled and then re-enabled, it is expected that the display name temporarily appears as Unknown User. This usually corrects itself automatically within about two weeks.
   • To speed up the update after re-enablement, the affected user should sign out of Teams completely and sign back in. After that, the display name typically updates within 72 hours.
2. Unable to communicate with any users (messages not delivered, cannot find or select users)
   This can occur if Information Barriers are applied incorrectly or too broadly in the organization. To check this, an IT administrator must:
   1. Run the following in Exchange Online PowerShell to see if the account is affected by an Information Barriers policy:

            Get-InformationBarrierRecipientStatus -Identity <user-identity>
      

      Replace <user-identity> with a unique identifier for the account (name, alias, email, or GUID). If an ExoPolicyId is returned, the user is in an Information Barriers policy.
   2. If a policy is applied, run:

            Get-InformationBarrierPolicy -Identity <ExoPolicyId>
      

      and review AssignedSegment, SegmentsAllowed, and SegmentsBlocked to confirm whether the policy is intentionally blocking communication. If the configuration is too restrictive, the admin must refine the segments/policies and reapply them using Start-InformationBarrierPoliciesApplication.

If the account is not in any Information Barriers policy and the “Unknown User” state persists beyond the expected time even after signing out/in, the organization’s IT admin should contact Microsoft Support for further investigation.

References:

• User appears as Unknown User in Teams
• Resolve communication issues in Information Barriers