Symptom
When using Outlook to access your standard mail account via IMAP, you may encounter the error message
The UID of a message changed unexpectedly. This typically indicates a server bug. Your program may not function properly after this.
The entire error dialog should look similar to the following screenshot:
Cause
You will typically encounter this problem if multiple clients are accessing the same mailbox simultaneously, especially if they are using a mix of IMAP and POP to connect.
This error may occur if a message has been deleted or moved but the associated event notification has not been sent to or received by all the clients connected to the mailbox.
Possible Solutions
If you are access the mailbox through multiple clients - for example on your PC and your smartphone - ensure both clients use IMAP and have similar synchronisation settings.
If this problem continues to occur, there may be a problem with Outlook's IMAP cache. If you remove the account from Outlook, then add it again this forces the cache to be regenerated.
As this problem only occurs with Outlook, an alternative solution may be to utilise another mail client like Mozilla Thunderbird, Opera Mail or eM Client.