Outlook re-downloading already downloaded email

[MOW]

We have a remote Outlook 2007 client (SP2) connecting to our Exchange 2003
SP2 via RPC over HTTP with cached exchange mode on.

The user is a laptop user who travels between offices regularly and often
works outside of the office. Typically when the user returns to their
primary office they find that on opening outlook there is 50MB-100MB of data
downloading to their Inbox but no new items. My guess is that Outlook is
re-downloading the emails that were already downloaded when the user was
outside the office.

No matter where they connect they download "full messages" not headers only
or headers then full messages. I have seen the issue occur once for one
other user. On the main affected user I've gone as far as replacing their
laptop and using a fresh profile to no avail.

Can anyone explain why this is happening or how I troubleshoot further to
determine exactly what is being downloaded?

 

[neo [mvp outlook]]

Anything you can see that might cause the OST to get regenerated each time
they connect? (e.g. Roaming profiles, Antivirus deleting/quarantine it,
..etc)

 

[MOW]

The entire OST file is not getting regenerated it is only a small portion of
the mailbox. The emails in the inbox don't actually disappear either
although sometimes some are duplicated at points in the re-download process.
There is no Roaming Profiles on the machine. Antivirus is run on a mail
gateway before it hits the exchange server.

It is a reasonably large mailbox 1.6GB which may contibute something.