When you first setup an Exchange task you might notice that we automatically discover all mailboxes within the organization including unlicensed mailboxes. Additionally, you might also notice a setting for 'Backing up unlicensed mailboxes' which is permanently enabled. The reason we've chosen to do this is because we want to make sure that we're doing the best we can to make sure there's no way that you can lose data that you'd want to backup. We feel so strongly about this that we will backup any unlicensed mailboxes that we can at no additional cost.
With the current implementation of M365 backups, we automatically discover all mailboxes when the task is created. Following the first scheduled backup job, we will be able to determine which mailboxes are licensed and which are not. Mailboxes which are unlicensed at the setup of the Exchange task cannot be backed up, unless they have previously been marked as shared. If any of those mailboxes are licensed in the future, they will automatically be backed up. If a licensed mailbox is marked as shared and then unlicensed, we will automatically continue backing it up, but your mailbox license count will be reduced so you don't have to pay for it. Just like with licensed mailboxes, all unlicensed mailbox backups include unlimited storage and retention.