Undo Send has just been added to Gmail. It doesn’t mean that you can reclaim an email that has been already sent. Instead, it is waiting for 5 seconds during which you can cancel the action of sending the email. After those 5 seconds, you mail will be sent, even if you close the window or if you computer reboots.
To enable the feature, go to Gmail Labs and click the “Undo” link.RELATED