If you've created a plan in Planner, then you are the group owner for the plan. The group owner can change a plan setting that will send group email messages to the group when a task is assigned or completed. By default, the group emails will not be sent to the plan's group mailbox in Outlook. If a member chooses to follow the group, then these activity emails will go to the plan member's inbox.
The following image is an example of a group email that you might receive.
Notes: Only group owners can turn off group emails. For more information about how to become a group owner, see Need to make someone else an owner of a plan?. Also, check out the following articles for more information:
Turn on group emails
By default task activity group emails are turned off. If group emails are turned on for your plan and you want to turn it off, just follow these instructions and de-select the setting.
From the plan board, select the 3 dots (. . .) to the right of the plan's name.
Choose Plan settings from the menu.
Select Send email to the plan's group when a task is assigned or completed, and then Save.