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.