“Immutability” is the industry-standard term for “preserving data in the system so that it is discoverable and cannot be destroyed or altered."
With Exchange Server 2016 with Enterprise CAL or Exchange Online Plan 2 (included in Office 365 E3 and Office 365 E5 plans), Microsoft enables organizations to preserve individual or all mailbox items for discovery natively, keeping those items within the Exchange infrastructure. This approach is called, In-Place hold.
One significant benefit of hold - as opposed to separate, read-only storage - is that items are preserved within the Exchange infrastructure. This preserves more of the information, including metadata, and makes management easier for IT admins. Users benefit because they can manage their mailboxes using the familiar Outlook interface. From an IT-perspective, In-Place Hold eliminates the necessity and complexity of maintaining a separate infrastructure and potentially storage for Exchange items.[...]