Alpha wrote:

To do this, I set both my laptop and desktop with "leave messages on
server". My idea was to copy the PST file from my desktop to my laptop
every time I'm going away, and later copy it back when I'm back home.
I keep all my received as well as sent mail for reference, so that's
why I want to copy the PST back and forth.

The problem is that when I copy the PST to the laptop, it does not
"remember" it has downloaded all those 1000's of messages that are in
the PST file - it starts downloading them again! I thought the
information about which messages were previously downloaded is saved
in the PST, but apparently this is not the case.

It's stored in the Registry.

There are a couple of ways to address it when using POP accounts. What I'd
do is to create a new PST, make it the delivery location, let it download
all the messages, then switch the delivery location back to the PST you
transferred. From then on, it should download only new messages.
--
Brian Tillman