Either your Outbox folder or sent items folder is likely corrupted.

First try the Outbox folder:
- Close Outlook Express.
- OE5+: Locate the file outbox.dbx in the mail directory and delete it.  You can use
Start, Find (or Search), Files or Folders, outbox.dbx to find it.  On Win2000 you need
enable the display of Hidden or System files in the folder options and XP you need to use
the advanced options to find Hidden or System files.  (You can also determine the location
by looking in OE at Tools, Options, Maintenance, Store Folder.)
- Open Outlook Express again.  It will create a new empty Outbox.

If you still have problems, then it's likely the Sent Items folder.
- Close Outlook Express
- Rename the file Sent Items.dbx to Sentold.dbx  You can use Start, Find (or Search),
Files or Folders, "Sent Items.dbx" to find it.   On Win2000 you need enable the display of
Hidden or System files in the folder options and XP you need to use the advanced options
to find Hidden or System files.  (You can also determine the location by looking in OE at
Tools, Options, Maintenance, Store Folder.)
- Open Outlook Express again.
- It will create a new empty Sent Items.
- The following program may be able to recover some of the messages from the Sentold.dbx
file.
 DBXtract - http://www.oehelp.com/DBXtract/Default.aspx
 With a very large DBX file, this can take hours or days.
- You can then open OE and select the Sent Items and drag the recovered EML files back
into it.