Inactive Mailboxes in Exchange Online (Office 365)

Understanding concept of inactive mailboxes in Exchange Online

When an employee leaves the organization we can remove their Office 365 account. after that employees’ mailbox is soft deleted and retained for 30 days, During this period, we can still recover the user’s Mailbox by undeleting the account. After 30 days, the data is removed permanently.

but if the organization wants to keep the data we can convert the mailbox into an Inactive Mailbox by placing the mailbox on litigation hold, or application Retention Policy feature in the Security & Compliance Center and after that removing the user account from office 365, the content of the user mailbox is stored until the hold duration gets expired. 

July 1, 2017, we'll no longer be able to create a new In-Place Hold to make a mailbox inactive. 

Only Litigation Holds and Office 365 retention policies can be used to create an inactive mailbox.

Litigation Hold and In-Place Hold

• Litigation Hold: Litigation Hold uses the LitigationHoldEnabled property of a mailbox. When Litigation Hold is enabled, all mailbox all items are placed on hold.
• In-Place Hold: we can use an In-Place Hold to preserve only those items that meet that the Search criteria of a query that we define by using the In-Place eDiscovery tool.

License Requirement:

Inactive mailboxes don’t require ongoing licenses.

But Exchange Online (Plan 2) license is required before the mailbox is deleted so that a Litigation Hold or Office 365 retention policy can be applied. Once the mailbox is deleted, the license that was associated with the mailbox will be available to assign to a new user. 

Managing inactive mailboxes

Following task can be done to manage the Inactive Mailbox.

• Recover an inactive mailbox: If a former employee returns we can recover the contents of the inactive mailbox. When you recover an inactive mailbox, the mailbox is converted to a new mailbox.
• Restore an inactive mailbox: If another employee takes on the job responsibilities of a former employee, or if another person needs access to the contents of the inactive mailbox, we can restore (or merge) the contents of the inactive mailbox to an existing mailbox.
• Delete an inactive mailbox: When you no longer need to retain the contents of an inactive mailbox, you can permanently delete it by removing all holds or Office 365 retention policies applied to the inactive mailbox. If a mailbox was made inactive more than 30 days ago, it will be marked for permanent deletion after you remove the hold.