Additional features, settings, or issues not covered by specific Microsoft Teams categories
Here is a way to send an e-mail using Planner when a task has been completed:
1) Use the Power Automate Template:
2) Configure your organization e-mail when prompted (to authenticate sending the notifcations)
3) Create your flow > Add the Team you want this to coordinate with > Choose the Planner name (Tasks in the example) > Choose who to send e-mail to > Click Save

4) Now when you complete a task you should receive an e-mail notification.

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