MS Outlook Error Message - The UID of a message changed unexpectedly

The cause of this issue normally relates to an issue/inconsistency in generating mailbox notifications and having all clients receive them.

Errors like these can occur if the notification events of message deletion are not being sent to or received by all clients connected to the mailbox.

Specifically, at the same time as an IMAP user is accessing the mailbox one (or both) of the following occurs:

1. The mailbox is being accessed using other mail access protocols and are making changes to the mailbox (e.g. HTTPMail, web mail, POP). An archiving or system agent that changes the messages/content in the mailbox could also cause the issue.

Note: If ALL users are using IMAP, this should not be an issue.

2. There is a problem whereby when an IMAP user is deleting messages the "notification messages" associated with deleting the message are not being received by other IMAP clients. 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. There is a bug in the IMAP client cache; and the IMAP client cache and its state does not match those on the server. The solution is to reconfigure the client IMAP support, hence having the cache regenerated. (There has been instances where Outlook's client cache can be out of sync with the server, potentially because of some sort of corruption).