Single-instance storage is a key requirement from customers who want to store users' mail centrally, on the server. With single-instance storage, if 100 users on the same server all receive the same message, only a single copy of the message is stored on the server, and 100 pointers to the message are placed in the users' mailboxes. Significant space savings and server performance gains can be realized with single-instance storage.
Microsoft Exchange Server has built single-instance storage capabilities into its information store design from the ground up, as opposed to stringing together existing databases. Single-instance storage is always in effect; it requires no special configuration or administration, and most important, it is intrinsic to the information store. When a message or user mailbox is deleted, the correct thing always happens; messages cannot be orphaned or lost. Pointers cannot get out of sync between files because everything is stored in a single file, and referential integrity is handled internally by the database engine.