The MessageID property returns a string which uniquely
identifies the message on the server. The identifier is assigned by
the mail server, and retains the same value across multiple client
sessions. This value is typically used when the client wants to leave
a message on the mail server, but does not wish to retrieve the
message contents multiple times. For example, the client can store
the unique ID for each message that it retrieves, but does not delete
from the server. The next time that it connects to the mail server,
it compares the unique ID of a message against the stored values. If
there is a match, the client knows that the message has already been
retrieved, and does not need to do so again.
This property requires that the server support the optional UIDL
command. If the command is not supported, this property will always
return an empty string. Note that the unique ID for the message is
not the same as the Message-ID header field in the message
itself.