Get the Message object corresponding to the given message number. A Message object's message number is the
relative position of this Message in its Folder. Messages are numbered starting at 1 through the total number of
message in the folder. Note that the message number for a particular Message can change during a session if other
messages in the Folder are deleted and the Folder is expunged.
Message objects are light-weight references to the actual message that get filled up on demand. Hence Folder
implementations are expected to provide light-weight Message objects.
Unlike Folder objects, repeated calls to getMessage with the same message number will return the same Message
object, as long as no messages in this folder have been expunged.
Since message numbers can change within a session if the folder is expunged, clients are advised not to use
message numbers as references to messages. Use Message objects instead.