I doubt you'll get anywhere trying to access the OST directly, since MS
goes to some trouble to make it inaccessible and unportable for security
purposes.
I've never tried CDO against a cached store, but I also can't think of any
reason it wouldn't work. The Outlook object model method would work, but
in my experience is better suited to doing thing that use the UI. Sounds
like what you're doing does not, so I'd encourage you to check out CDO.
Handy site to know about if you don't already:
http://www.cdolive.com/
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David L. West
http://www.deskoptional.com