If you see the following message in Outlook, or any other email client:
"The uid of a message changed unexpectedly. This typically indicates a server bug. Your program may not function properly after this"
This is generally caused by an email account using both POP and IMAP to collect emails.
These are two completely separate email retrieveal systems that conflict with each other and the should NEVER be used on the same mailbox....especially at the same time. What often happens is that while one protocol is downloading emails and deleting them from the system (POP), IMAP is attempting to synchronize they emails and "marking" them in some way so it can keep track of the snychronisation. It's these conflicting processes that are causing the error.
If this is the only error message you have received you should consider yourself lucky in some respects. The most common outcome of using these 2 different protocols on the same mailbox is that it ends up corrupting the mailbox so badly that the mailbox stops working and the emails cannot be recovered.
The best thing is to choose a single protocol (POP or IMAP) to collect your emails, and stick to that protocol at all times.