Microsoft's support site contains great information on how to create and use Microsoft Planner.

Important Planner Note: If you are a member of a plan, please read the below information to prevent missing comments during your plan lifecycle. 

Warning: Deleting the group email thread associated to the task will also delete all existing comments for the task. Planner group inboxes appear in Outlook as shown below. Many users are unaware of these groups, but it is important to notify plan members to never delete emails from these groups that are related to a plan task. Planner comments are not recoverable once deleted.