The UnreadMessages property returns the number of messages
which do not have the SEEN flag in the current mailbox. This value is
not the same as the number of recent messages in a mailbox, which is
based on when the message was stored in the mailbox. To obtain a list
of messages that have not been read, use the SearchMailbox
method with UNSEEN as the search criteria.
It is possible that a message may be flagged as seen if it
has been previously accessed by a different mail client. For example,
a client may retrieve a message from an INBOX mailbox using the POP3
protocol, which would cause that message to be flagged as seen. This
behavior is server dependent, and is most commonly found where the
mail server supports both the POP3 and IMAP4 protocols.