I assume you are trying to take a mail archive with you, on a laptop etc., but retain a backup on the network? If so it is best to enable offline use in the client and let exchange take care of all the sync'ing, if you really must do it you need to manually copy the file from your local drive to the network as often as you can - Outlook must be closed for it to work. Microsoft do/did a small program that automates "Personal folders backup" called pfbackup. I am not sure if you can still download it, It was a little unstable but worked most of the time.