Hello,
could it be that your sent items folder is full? This happened to me- what you could try is moving all items from sent items into a newly created folder, e.g. old sent items, then close outlook express, go to the maintenance store folder where email .dbx folders are stored and delete sent items.dbx. The folder is regenerated when you start outlook express.
For some reason, even when folders are emptied of email, it doesn't appear to decrease in size. It's like it holds onto an image file or something
Good luck