GMail Introduces Undo Send

Undo Send has just been added to Gmail. It doesn’t mean that you can reclaim an e mail that has been already sent. Instead, it is waiting for 5 seconds during which you can cancel the action of sending the mail. After those 5 seconds, you mail will be sent, even whether you close the window or whether you computer reboots.

To enable the feature, go to Gmail Labs and go the “Undo” link.