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Consider the following scenario. You move a mailbox from an Exchange Server 2003-based computer to an Exchange Server 2007-based computer. A POP3-based client or an IMAP4-based client downloads an e-mail message again in this scenario, even though this e-mail message was already downloaded when the mailbox was located on the Exchange Server 2003-based computer.
The POP3 protocol uses the Uidl command to determine whether a message was downloaded. The IMAP4 protocol uses the UID command to determine whether a message was downloaded. Both commands rely on the Unique ID (UID) property of the message. When you move a mailbox from Exchange Server 2003 to Exchange Server 2007, the corresponding UID of the message is changed. This causes the problem.
To resolve this problem, install Exchange Server 2007 Service Pack 2 (SP2). You can access the following Microsoft Web site to download Exchange Server 2007 SP2:
Note Exchange Server 2007 SP2 does not fix duplicates that already exist on the client. A complete new sync of the client is necessary for this. However, if Exchange Server 2007 SP2 is installed before the mailboxes are moved, no duplicates appear on the POP3/IMAP4 client.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about Internet protocols, visit the following Microsoft Web site: