Should email providers add "UNSEND"?
cchooper: Some clients (e.g. Outlook) let you put a delay on your outgoing email, so you can cancel it if you change your mind in time.And the email recall feature of Outlook/Exchange is also quite good. It even tells you how many emails couldn't be recalled (because people have already read them). It will only work within an organisation though, as it is a feature of the Exchange server.