Make sure you have a backup of the mailbox. Go to Outlook profiles by going to Settings -> Control Panel -> Mail -> Show Profiles. Select the name from the profile list and click on Properties -> E-mail Accounts -> View or Change -> Change -> More Settings. Click Advanced and make sure that Cached Exchange mode is NOT checked. If it is, uncheck it and save the profile. You may need to delete the profile and create a new one.
I have been having the exact same problem, and this solution offered is not helping. I've tested it on 17 accounts, and I've had the problem on 9 of those 17. I've also noticed that the issue is occurring on only Microsoft Outlook 2003, not Outlook 2000 and 2007.
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This was first published in October 2005