Exchange supports single instance message storage for messages sent to mailboxes in the same database. Only one instance of the message is stored in the database.
If the message is sent to mailboxes in different databases, the message is written once to each database.
Single instance storage is not maintained when a database containing a message is moved to a server containing a database with the same message.
Tip To maximize single
instance message storage, place similar users in the same mailbox
store, such as users in the same department who use Reply All or
users who frequently send large attachments to one another.