Lost emails in Outlook Express


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What can cause some (not all) emails to not be listed in the Inbox in Outlook Express 6.0, and is there a way of repairing this?
I have seen this occur on two different machines in the last two days; both running XP Home, one with SP2, fully updated, and one running SP1. On the SP1 system, emails have vanished from an approx. one-month period. On the SP2 system, all emails since May 2006 are no longer listed. I have checked to make sure they are not in the Deleted Items folder or any other folder (the SP1 system has an incredibly complex folder structure within Inbox, making it not feasible to check this).

I can supply additional info if necessary; but any ideas as to the cause and, more important, the solution would be greatly appreciated. Thanks in advance