When you "delete" email in Outlook Express, they aren't really deleted! If you ever upgraded one version of Outlook Express to a newer version you probably experienced that: after upgrading you notice a lot of old email you thought you had deleted "returned".

Outlook Express just removes the "pointers" to the actual email from the index, but to actually remove the email itself, you need to run the Compact Folder function.

You can access this function from the Outlook Express menu: File -> Folder. You can choose either Compact or Compact all Folders, the difference is that the first one only compacts the folder currently selected (see picture 1).

Picture 1