Remember to only export your mailbox just before you move as it rips all of your mail from exchange to the .pst file. Alternatively, you can export your mail then change the delivery of new mail to the .pst file rather than the exchange mailbox.You should also ensure you have a forwarder on the old exchange box to your new exchange email account or you will have mail going missing. I don't really know the difference between a .ost and a .pst file (other than the source) but you are right in thinking it is daft to create a file that is already there. That's Microsoft for you!