This method is called internally by the wodMailbox
whenever you access some message for the first time. It has
to do it, because it didn't allocate memory when you
initially Locked file.
Potentially large messages (or with large attachments)
should be Unloaded after
usage, to free up computer resources. This applies only to
extremely large messages (around 100mb or more), or if many
messages are loaded in memory at once. wodMailbox will not
free them after usage - because it doesn't really know
when you will stop using the message. It will do so when
you Unlock the
mailbox.