Keepit does not back up inactive mailboxes.
A mailbox is considered to be inactive after the following two steps are taken:
1) The mailbox is placed on Litigation Hold or has an Office 356 retention policy applied to it;
2) The mailbox or corresponding Office 365 user is deleted.
A mailbox that is placed on Litigation Hold or has an Office 356 retention policy applied to it but has NOT been deleted is considered to be active. We do back up such active mailboxes.
For more about mailboxes on Litigation Hold, see Does Keepit back up data on Litigation Hold?