Exchange single-instance storage and its effect on stores when moving mailboxes

SUMMARY

Microsoft Exchange Server 5.5, Microsoft Exchange 2000 Server, Microsoft Exchange Server 2003, and Microsoft Exchange Server 2007 maintain single-instance storage of messages.

If a message is sent to one recipient, and if the message is copied to 20 other recipients who reside in the same mailbox store, Exchange Server maintains only one copy of the message in its database. Exchange Server then creates pointers.

These pointers link both the original recipient and the 20 additional recipients to the original message. If the original recipient and the 20 additional recipients are moved to another mailbox store, only one copy of the message is maintained in the new mailbox store.

The new mailbox store can be on another server in the same site or in an administrative group. If the server is in another site, single-instance storage is retained only if you use one of the following items:

• The Move Mailbox wizard in Microsoft Exchange Server 2003 Service Pack 1 (SP1)
• The Move Mailbox wizard in Exchange Server 2007
• The move-mailbox PowerShell cmdlet in Exchange Server 2007

The Exchange Server 2003 SP1 Move Mailbox wizard introduced a new Cross Administrative Group Move feature that lets you move mailboxes across administrative groups.

Earlier versions of Exchange Server require that you use the Exmerge tool. This process causes the loss of single-instance storage.

For more information about the Exmerge tool, click the following article number to view the article in the Microsoft Knowledge Base: For example, assume the following configuration:

Server 1 Mailbox Store 1: Users A, B, and C
Server 2 Mailbox Store 1: User D

When User A sends the same message to User B, User C, and User D, Exchange Server creates 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 User A's Sent Items folder is a single instance of the messagesthat is in the Inboxes of User B and User C. Because User D is on a different server, Exchange Server sends a message to server 2 that will be stored on that server.

Note If a message is sent to a user who is on the same server but on a different mailbox store than the sender is on, that mailbox store will also contain the message.

During a move of users, Exchange Server checks every incoming message against its own internally assigned IDs. These internal IDs are not viewable with a client. If Exchange Server determines that the incoming message is a duplicate, it just sets up a pointer for that existing message that points to the newly moved user. Assume that User B is moved to server 2. Exchange Server will create a single instance of the message for User B based on the copy of the message that User D has. This is true if user D has not deleted the message. If User D has already deleted the message, Exchange Server 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 Server 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 Server will maintain a single instance of the message for User B and User C. This is based on the message that is in the Inbox of User D. If User D has already deleted the message, Exchange Server will maintain a single instance of the message based on the message that is in User D's Deleted Items folder. If User D has already emptied the Deleted Items folder, Exchange Server creates a new message and sends it to server 2.