Undeletable message in Windows Mail

For a few days I had a message in my outbox that couldn't be deleted (it had been send correctly however).

In the preview window there was an error saying "Windows Mail encountered an unexpected problem while displaying this message. Check your computer for low memory or low disk space and try again."

Trying to delete the message resulted in "An unknown error has occurred." Since that message was stuck in my outbox, I was unable to send any more emails using the account used for that message.

I spend some time reading articles in microsoft.public.windows.vista.mail, but nothing helped.

So I decided to export all my messages and I closed Windows Mail. I renamed the WindowsMail.MSMessageStore file and I started Windows Mail again. It detected all my previous folders and restored all my mails and subfolders, the only thing I had to do was to move them out of the imported folder...