With Exchange Server 2010 SP1 and Exchange Online, Microsoft enables organizations to immutably preserve mailbox items for discovery using an in-place archive, keeping those items within the Exchange infrastructure. While the technology used to achieve immutability is different than sending items to a separate, read-only archive, the Exchange approach achieves the same results—preserving all data in a way that is tamper-proof and discoverable—while at the same time conserving more of the relevant data, reducing IT complexity, and delivering a seamless user experience.

Achieving Immutability with Exchange Online and Exchange Server 2010