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, and Exchange Online, 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, preserving more of the information including metadata. Users benefit because they can manage their mailboxes using the familiar Outlook interfaces. From an IT-perspective, In-Place Hold eliminates the necessity and complexity of maintaining a separate infrastructure for Exchange items.
Exchange gives organizations the flexibility to choose the architecture that can help meet their immutability requirements whether that is on-premises, online, or a hybrid of both, and supports the ability to store archived items in a separate physical location.