This is a very common problem with Outlook Express when you have lot of email exchange on regular basis. Outlook Express works fine till it's folders size reaches to 2GB. Beyond that point, Outlook starts having problem.
This happens because when you send a message, Outlook Express copies the message to sent folder and if there is not enough free space in Sent Folder to copy the message, then Outlook Express leaves the message in the Outbox and it is sent repeatedly again and again.
Try following tips to get rid of this problem in Outlook Express:
1. Create a New Folder called "Old Sent" and move all messages from Sent Folder to Old Sent.
2. Empty "Deleted items" Folder on regular basis.
3. After you move everything from Sent folder, delete Sent and Outbox folder and then close and open the Outlook Express.
4. Turn off email scanning in your anti-virus program for testing phase and then you can turn ON again depending upon the observation.
After you are done with all the above, compact your all folders manually.
1. Make sure that no folders are open
2. Then go to File and Select "Work Offline"
3. Go to File again and then Folder -> Compact all folders
Caution: Don't do anything until the compacting action is completed.