SYMPTOMS

Microsoft Outlook displays an error message - The UID of a message changed unexpectedly.

CAUSE

This occurs when Microsoft Outlook gets information about a message from the server which indicates that the client is out of sync with the server. The cause of this issue is due to Outlook not being made aware of changes to the mailbox from other activities while it is connected, which mostly would occur if a message is removed from a mailbox without the client being notified.

Check the following to resolve:

1. Make sure no messages are being removed from the mailboxes from outside the mail services. If messages are removed with other protocols, such as webmail or ActiveSync, these will notify the IMAP client,  but deleting or moving messages directly in Windows Explorer will cause the error message.

2. Notification messages may not be received by Outlook. This could be caused by any of the following:

2A). A client firewall IMAP proxy software is intercepting the communications associated with IMAP and is not allowing the notifications to be received properly.

2B). There is some other network constraint preventing these notifications from being received.

3. If you are running a cluster for MailEnable, check to make sure RPC is enabled and working between the servers. RPC is used in order for one server to notify another of a change.