Have you ever sent an email, and just as it was going on its merry way, you realize you misspelled something or you sent it to the wrong person. Well, now you can take advantage of that delay to “undo” the message. Just enable the feature in Gmail Labs in Settings. It only works during that 5 second delay between the time you hit send and the time that Gmail actually sends the message.
Read the article: Gmail Undo Send