To enable Undo Send:
1.Click the gear in the top right .
3.Scroll down to "Undo Send" and click Enable.
4.Set the cancellation period (the amount of time you have to decide if you want to unsend an email).
5.Click Save Changes at the bottom of the page.
If you had Undo Send turned on in Gmail Labs, your Undo Send setting will be on by default.
Note: To give you time to undo, Gmail delays sending the message for a few seconds. So if you don't select "Undo" within the time limit, your message will be sent. "Undo send" may not work if you experience connection issues.