An interesting question came up in the newsgroups recently: “I have a dual boot setup and I want to be able to use the same .pst without having to download same messages twice and I'm trying to figure out where Outlook stores information about the messages it has already downloaded. I've used Regmon and Filemon but I wasn't able to figure it out.”
You’ll never find it using those programs (and you won’t be able to stop each profile from re-downloading the messages, short of not leaving them on the server). “Outlook Mailbox Manager” stores the last downloaded Message ID. It’s a hidden message in the Inbox and is tied to the computer and profile that created it. Each account that opens the pst will create its own hidden Mailbox Manager message.
To satisfy your curiosity, you can view it using MFCMAPI. Logon to your profile, right click on the default pst and choose open store. Expand the Root mailbox, IPM_Subtree and right click on Inbox and choose Open Associated Contents. This is where Outlook stores rules, RSS feeds and other hidden data. As long as you don’t delete anything, its safe to browse around and see what Outlook hides inside your mailbox.