I would suggest that your Outbox is corrupted. Open OE and go to Tools menu - Options - Maintenance tab and click on Store Folder. Place the cursor on the address and right-click - select all. Then right-click again and Copy. Close down Options box and OE - then go to Start - Run - and right-click in run box - select Paste. The box that opens is all your .dbx files. Find the Outbox and delete it. Close the box - then open OE again, and there will be a new Outbox created. Hopefully it should now work. IMPORTANT - make sure OE is closed while you are using the Run box and deleting.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been