have you tried cleanup, if you have then and it did not work you could try this.
change the windows files and folder options to (show hidden files).via Organize/folder and search options/veiw/show hidden files.
Then navigate to (C)if thats your active drive.
then navigate through users/your accountname/AppData/local/microsoft/windows mail/local folders/outbox......
the only file in here should be winmail.fol.
delete anything else.