When planning a migration to Office365 from an on-premise Exchange server, one of the pre-flight tasks is to reconcile the mailbox list to verify the number of licenses needed. In very general terms, Microsoft requires a license for any mailbox that’s logged into by a user. The good news is that this often exempts a license being necessary for items like resource mailboxes, room mailboxes, and shared mailboxes, except when they are over their storage quota of 50 gigabytes (GB). More detail can be found in this Microsoft blog regarding subscriptions and billing.
However, other services may require licenses for mailboxes that Microsoft does not. For example, when migrating data from an on-premise Exchange server to Office365, migration tools will require a license for resource mailboxes, room mailboxes, and shared mailboxes. Similarly, solutions that backup Office365 data for redundancy also require licensing for these mailboxes, as well.
Questions, comments, concerns? Yep… we’ve heard ’em, and we’re here to help make sense of this licensing soup.