Gmail undo send option

According to official gmail blog, Google introduce new service for their gmail users, where users can hold their email just for five seconds before sending and can call back for correction. Google now offer an "undo send' features in gmail, which can hold message just five second in a separate windo while they hit send option.
Gmail's send option now have 'undo' option link. By clicking on 'undo' will return a message to its draft and it can be send after correction.
But user can not call back that message which was already sent. This option is only when you hit send button and for five second you can recall that message. To turn it on, sign in to your Gmail account, click "Settings," click on the "Labs" header and scroll almost all the way down the page to "Undo Send."