Exchange 2010 SP1 and Exchange Online (Office 365) Immutability Whitepaper
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