There were about 6000+ emails in the sent box. I deleted about 3000 to see if was a size issue and nada.
Maybe the .dbx file of your sent box is damaged.
You could try the following:
Export your messages from the sent box to another mail application. Then copy the .dbx file of the sent box to a different folder - Outlook Express should create a new .dbx file. Then import the messages again and it should work.
Michael
Unsolved 
