How to Find Unused Exchange Online Mailboxes

Finding and removing unused Exchange Online mailboxes used to be a good way to keep Office 365 licenses costs under control. Given the widespread use of Exchange Online as part of bundles like Office 365 and the effect of Teams on email for internal communication, looking for unused mailboxes might not be so important now. In any case, the techniques of looking for evidence of mailbox under-use are interesting and useful for tenant administrators to understand, which is why we have this article!