"Migrating" email in Outlook Express

I'm helping out a small office on a p2p network, using Windows 98 & ME and Outlook Express. Their email is IMAP, *not* pop3. This allows each user to see each other's email. On each worstation, in OE, the local folders pane has a tree with the inbox created by OE at the top, and then down the tree are 4 other inboxes, 1 for each workstation.
The issue is this: when mail comes in to the default account on the particular workstation, it appears in the approprite inbox (employee#1, for example). However, shortly afterwards, and without prompting, the mail "migrates" to the topmost inbox (the one created by OE), and can no longer be seen from any other workstation.