2.1.3 [RFC3501] Section 18.104.22.168, Combination of Mailbox Name, UIDVALIDITY, and UID MUST Refer to Single, Immutable message
The specification states: "The combination of mailbox name, UIDVALIDITY, and UID must refer to a single immutable message on that server forever. In particular, the internal date, [RFC2822] size, envelope, body structure, and message texts (RFC822, RFC822.HEADER, RFC822.TEXT, and all BODY[...] fetch data items) must never change."
Although Microsoft Exchange Server adheres to this rule, other protocols have access to these messages, and some of these protocols modify message properties such as the message body. Changes to the message in this way result in a new UID value for the message.