Did you hit “Send” too soon? Accidentally clicked “reply all”, or just wish you’d not sent that last mail?
If you’re using Gmail (most of us are, it seems), you can stop messages from being sent for a few seconds after hitting the send button.
Log into your Gmail account. Choose the Settings icon:
Choose the Labs link (top row). Find the Undo Send option in the list on the middle of the page:
Click Enable. Click Save Changes at bottom.
Remember, this is an experimental feature.
So it might change over time. If (or when) it breaks, and you have trouble loading your inbox, the good news is there’s an escape hatch. Use https://mail.google.com/mail/?labs=0.