Hi @IBN
To check what happens when the meeting invitation email is sent from Microsoft Teams, please contact your admin to run a message trace for this mail flow. In addition, it is also recommended to contact the internal user to check their Deleted folder or Junk email folder.
Besides, please check if their settings has turned on the meeting started notification like the following picture.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.