Some Exchange Online mailboxes are quite small (2 GB for frontline users). Tenant administrators might want to monitor mailbox usage to make sure that quotas aren’t unexpectedly exhausted. This post explains how to use a PowerShell script to calculate the percentage of mailbox quota used and highlight the problem if a threshold is passed.
Although Office 365 supervision policies are intended to monitor a subset of user communications, usually involving specific groups of people, you might want to use a policy to monitor all email. In that case, how do you make sure that your policy has everyone in scope? The problem is that supervision policies don’t support dynamic distribution lists, so you need to do some work to build and maintain a distribution list containing all user mailboxes.