Microsoft Server Exchange 2010 provides several solutions for failover redundancy. These solutions include the following:

High availability and site resilience: You have the option of deploying two Active Directory (AD) sites in separate geographic locations or stretch a single AD site between the two locations, keep the mailbox data synchronized between the two, and have one of the sites take on the entire load if the other fails.

Online mailbox moves: In an online mailbox move, end users can access their e-mail accounts during the move. Users are only locked out of their accounts for a brief time at the end of the process, when the final synchronization occurs. Online mailbox moves are supported between Exchange 2010 databases and between Server 2007 Service Pack 2 (SP2) and Exchange 2010 databases. You can perform online mailbox moves across forests or in the same forest.

Shadow redundancy: Shadow redundancy protects the availability and recoverability of messages while they are in transit. With shadow redundancy, the deletion of a message from the transport databases is delayed until the transport server verifies that all the next hops for that message have completed. If any of the next hops fail before reporting successful delivery, the message is resubmitted for delivery to the hop that did not complete.