The folder index and message list may have somehow gotten out of sync or the index is damaged. If so, easy fix: - The first thing to try is to compact the folder (right-click on the folder name and choose "Compact". - If that doesn't fix the problem, go on to the next possible fix, which is to Repair the folder. This is safe, but you might find that some messages you deleted earlier now re-appear. Right-click on the folder name, Properties, General Information, and click the "Repair Folder" button.