XADM: How Moving Mailboxes Affects Single Instance Storage

SUMMARY

Microsoft Exchange maintains single-instance storage of messages even when users are moved between servers in a site.

For example, assume the following configuration: When User A sends the same message to User B, User C, and User D, Exchange makes a single instance of the message on server 1 for all three users, because User A, User B, and User C are on the same server. Even the message that is in the Sent Items folder of the sender (User A) is a single instance of the messages that are in the Inboxes of User B and User C. Because User D is on a different server, Exchange sends a message to server 2 that will be stored on that server.

Important On Microsoft Exchange 2000 Server, if a message is sent to a user who is on the same server but on a different mailbox store than the sender, that mailbox store will also contain the message.

Now assume that User B is moved to server 2. Exchange will create a single instance of the message for User B based on the copy of the message that User D has, if user D has not deleted the message. If user D has already deleted the message, Exchange will create a single instance of the message based on the copy of the message that is in User D's Deleted Items folder. If User D also deleted the message copy in the Deleted Items folder, Exchange creates a new message and sends it to server 2.

The same process occurs if User B and User C are moved to server 2. Exchange will maintain a single instance of the message for User B and User C, based on the message that is in the Inbox of User D. If User D has already deleted the message, Exchange will try to use the copy in User D's Deleted Items folder. If User D has already emptied the Deleted Items folder, Exchange creates a new message and sends it to server 2.