First, what is an inactive mailbox? Simply put, an inactive mailbox is one that is no longer used because the mailbox owner (employee) has left the company, leaving the mailbox orphaned. It is very common for mailboxes to remain in the Exchange system after an employee leaves the company for several reasons including standard company practices, lack of company processes, legal reasons i.e. anticipated lawsuits, and regulatory concerns. These orphaned mailboxes can become an issue later, for example if an organization decides to move their email system to the cloud.