Inactive mailboxes occur when an employee leaves the company. To save money, the company will remove the O365 license for that user's mailbox to avoid paying for the user. 

The process of removing the license will sever the link between Active Directory and O365 which causes 3rd party software to be unable to collect from the inactive mailbox.

Before a company removes the license on a mailbox, they can place the mailbox on hold in O365. This allows the held data to be searchable within the O365's "Security & Compliance Center."

Veritas has confirmed this behavior with Microsoft Support.