Tools>Options>Send tab. If there's already a check in the first option,
uncheck it, apply, then re-check it and apply.

Select the Sent Items folder then View, Current View and make sure that
you are not hiding read messages.  Also make sure that Group Messages By
Conversation is not checked.

Select Tools, Options, Send and make sure that you have checked the
option to save a copy in the Sent Items.

If the above are OK, then your Sent Items may be corrupted.
- 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.

General OE Caveats:

- Don't use Inbox or Sent Items to archive messages.  Move them to local folders created for this purpose.

- Empty Deleted Items folder daily.

- Disable Background Compacting [N/A in SP2] and frequently perform a manual compact of all OE folders while "working offline".  More at http://www.insideoe.com/files/maintain.htm.

- Do not attempt to close OE via Task Manager or shutdown your machine if Automatic Compacting is taking place (WinXP SP2 only).

- Your anti-virus application's email scanning feature can also cause  corruption.  Disable it.  It provides no additional protection.