Did you sent a mail to a wrong person by mistake, Puzzled ?? Do you need a feature of undo send? Google has introduced a new feature of Undo Send in GMail where you can stop sending of messge in say about 5 second delay between you hit the send button and GMail actually send the mail.
To enable this feature in your GMail, login to your gmail account, check labs section in Gmail, or goto settings -> click Labs -> enable the Undo Send feature and here you go.