Exchange Online makes it possible for you to preserve the contents of deleted mailboxes indefinitely. This feature is called inactive mailboxes. A mailbox becomes inactive when an In-Place Hold is placed on the mailbox before the corresponding Office 365 user account is deleted.
You do not have to pay for keeping inactive mailboxes.
The contents of an inactive mailbox are preserved indefinitely. This allows administrators, compliance officers, or records managers to use the In-Place eDiscovery feature in Exchange Online to access and search the contents of an inactive mailbox. Inactive mailboxes can't receive email and aren't displayed in your organization's shared address book or other lists.
Note: If a hold isn't placed on a mailbox before it's deleted, the contents of the mailbox won't be preserved or discoverable. The mailbox can be recovered within 30 days of deletion, but the mailbox and its contents will be permanently deleted after 30 days if it isn't recovered.
To make a mailbox inactive, it must be assigned an Exchange Online (Plan 2) license or have an Exchange Online Archiving subscription so that an In-Place Hold can be placed on the mailbox before it's deleted.
1. Access the contents of an inactive mailbox
After you make a mailbox inactive by placing the mailbox on In-Place Hold and then deleting the corresponding Office 365 user account, you can access the contents of the inactive mailbox by using In-Place eDiscovery in the Exchange admin center (EAC). When you search an inactive mailbox, you can create a keyword search query to search for specific items or you can return the entire contents of the inactive mailbox. You can preview the search results, copy the search results to a discovery mailbox, or export the search results to an Outlook Data (PST) file.