GMail added another new feature in Gmail Labs.It’s called “Undo Send,” and as the name suggests, it lets you take back a sent email, as long as you act quickly enough.If you addressed an e-mail to the wrong person, let slip with an embarrassing typo or simply said something you really, really shouldn't have, Undo Send can be a lifesaver.Gmail’s undo feature which gives you a 5 second window to stop, cancel and undo a sent email.It is not activated by default, and you need to enable the feature for it to work.
Gmail users can enable the new Undo Send feature by going into their Labs settings and turning it on -- click on the little green flask at the top of the screen, or go to Settings and click on the Labs tab.
Gmail > Settings > Labs > Undo Email > Enable > Save Changes
Google in its blog is calling it a “panic” button - which is really all it is. It gives you the chance, if you’re quick enough to change your mind - but only if you’re quick enough.