After migrating mailboxes from one Exchange server to another, the size of the mailboxes may strangely start increasing by themselves. One possible explanation for this peculiar behavior is that some types of mailbox-move actions can break Exchange Server's single-instance storage.
Single-instance storage is one of Exchange Server's major selling points. If you send an email to one recipient and copy it to 10 others, only one copy of the actual message is retained by Exchange Server; the other recipients get a pointer to the original message.
On the client side, there's no difference between the pointer and the original email; all a user sees when running Microsoft Outlook (or Outlook Web Access) is the message itself. This reduces the total amount of storage Exchange Server needs for messages, especially when company-wide distribution lists or similar mechanisms are used a lot.
Exchange Server single-instance storage is store-centric, meaning it will store one copy of a given message for each store that it has to send it to. Moving Exchange Server mailboxes between stores on the same server preserves single-instance storage, since Exchange Server knows intuitively how to preserve the single-instance pointers.
But, if mailboxes are moved to an Exchange server in another site, the pointers are no longer valid. Each individual copy of the email has to be recreated and moved. If you have a message sent to multiple users who have Exchange Server mailboxes in different stores, a copy of the message will be created for each store.
The best way to move Exchange Server mailboxes between stores, servers and administrative groups is through the Exchange Server 2003 SP1 Move Mailbox Wizard. It has a Cross Administrative Group Move function that preserves single-instance storage when moving Exchange Server mailboxes. In the past, you had to use EXMERGE to do this, which broke single-instance storage.
Finally, as far as I know, there is no way to recreate single-instance storage on a message once it's been "split" during a move. So proceed with caution when moving mailboxes to avoid this problem in the first place.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.
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Related information from SearchExchange.com:
Tip: Preparing for an Exchange Server mailbox migration
Tip: Moving mailboxes across administrative groups in Exchange 2003
Learning Guide: Exchange Server migration tips and advice
Reference Center: Exchange Server mailbox management tips
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