Outlook Express was partly responsible for my losing a number of emails, too. Silly me, I allowed OE to compress my folders when prompted, but then deleted the contents of the recycle bin and shut down the PC a bit fast when I thought it had finished doing this- it evidently had not.
Long shot, but check your recycle bin.
Then do what I did, and install Mozilla's Thunderbird. Seems to be much more reliable, and can be configured easily to leave mails on the server .