Personal Outlook and Teams Unknown User attendee

Check the Microsoft account’s primary alias first: if it is a non-Microsoft address and Teams sends invitations from an unavailable generated @outlook.com address, create a working Outlook.com alias and make it primary; if the primary alias is already Microsoft-hosted, escalate the post-send attendee mutation to Microsoft Support.

If the primary alias is a non-Microsoft address

Similar Teams Personal cases document that accounts using third-party addresses can send invitations through a generated @outlook.com identity, causing replies or NDRs. Changing the primary alias addresses that sender-address problem, although the available documentation does not confirm that it will also remove the visible Unknown User entry.^1^

1. Sign in to the Microsoft account alias-management page: https://account.live.com/names/manage
2. Select Your info → Manage how you sign in to Microsoft.
3. Select Add email.
4. Choose the option to create a new Outlook.com address.
5. After it is created and verified, select Make primary beside it.
6. Keep the current subscription/sign-in address as a secondary alias; do not remove it during testing.

Aliases share the same Microsoft account, password, subscriptions, contacts, and storage. An existing Outlook.com, Hotmail, Live, or MSN address already associated with another Microsoft account cannot be added as an alias.^2^

After the change:

1. Sign out of Teams Personal on both desktop and web.
2. Sign back in using the new primary Outlook.com alias.
3. Create a new test meeting with one known recipient.
4. Reopen it after sending and check:
   • whether Unknown User was inserted;
   • the sender/organizer address shown in the invitation;
   • whether an NDR is produced.

Use a new meeting as a diagnostic comparison; the documentation does not establish that changing the alias rewrites previously created meetings.

If the primary alias is already a working Microsoft address

Treat this as a server-side calendar/identity mapping issue rather than applying the third-party-alias workaround.

1. Sign in to Outlook.com with the same personal Microsoft account.
2. Open Calendar, locate the affected meeting, and inspect its attendees there.
3. If the meeting originated in Outlook, modify its attendees in Outlook.com. Teams Personal can display Outlook-created meetings, but attendee changes for those meetings must be made through the account’s Outlook calendar.^3^
4. Preserve the following evidence:
   • the complete NDR, including diagnostic/error codes and message headers;
   • a screenshot before sending and another after reopening;
   • the unexpected Outlook.com address, partially redacted;
   • whether Outlook.com also shows the attendee;
   • the date, time, and time zone of a new reproduction.

Then contact Microsoft Support through the Microsoft account support route and report that the attendee is inserted after the event is saved by the service, reproduces in Teams desktop and web, is absent from Tracking, and returns after removal. Microsoft’s alias-management page provides the public Contact Support path.^2^

The documented “external user shown as Unknown” behavior applies to external users displayed in Teams admin-center call analytics; it does not explain an address being inserted into a Teams Personal calendar event after sending.^4^

References

1. Wrong email in Teams - Microsoft Q&A
2. Change the email address for your Microsoft account
3. [Web](https://support.microsoft.com/teams/personal/meetings-calls/schedule-a-meeting-in-microsoft-teams-personal#share-your-meeting-details)
4. External users shown as Unknown in meeting details - Microsoft Teams