The MailboxUID property returns an integer value which
uniquely identifies the mailbox and corresponds to the UIDVALIDITY
value returned by the IMAP server. The actual value is determined by
the server and should be considered opaque. The protocol
specification requires that a mailbox's UID must not change unless
the mailbox contents are modified or existing messages in the mailbox
have been assigned new UIDs.
An application can use the MailboxUID property value in
combination with the MessageUID property in order to uniquely
identify a message on the server. However, the application must take
into consideration that the IMAP server can reassign new message UIDs
when the mailbox is modified. If the mailbox and message UIDs are
being stored on the local system to track what messages have been
retrieved from the server, the application must check the UID of the
mailbox whenever it is selected. If the mailbox UID has changed, this
means that the UIDs for the messages in that mailbox may have
changed. The client should resynchronize with the server, and update
it's local copy of that mailbox.