this is for outlook express 5 and i know this works if you have a huge file in your outbox, i've changed a few of the steps for you so you could create a test account and try to see if it works on the inbox aswell.
first goto your inbox and move any mail that you need to a different folder, now go to your outbox and stay there. Go to tools > options and then it that window go to the maintenance tab and click the button that says folder store. A window will pop up with a location something like c:\windows\ application\data\indentities\{?????????-????-????-????-???????}\microsoft\outlook express, you need to copy this. close outlook, go to windows explorer and paste it into the location bar, you should now see loads of .dbx files with one called inbox.dbx, delete this, dont worry the indox recreates itself. Open outlook and go to your inbox which should be empty, copy the mail you saved to another folder back to your inbox.
hope this helps
miguel