When a new message is received in Teams, the user always get a notification and there is a pop-up message with preview.
This can be turned off in two ways:
1. If the notifications needs to be turned off during the meetings only, the status should be changed to "Do not disturb" and then there will be no notifications/pop-up messages during the meetings.
2. You can turn off the notifications/preview from the settings in Teams app.
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