In your Gmail account, click the gear icon in the upper-right corner and choose “Settings.” On the “General” tab, scroll down and find “Undo Send.” Click the checkbox for “Enable Undo Send” and choose a cancellation period of 5, 10, 20 or 30 seconds. (This is how long you have to call it back before it gets delivered.) Once you have that set, scroll down to the bottom of the screen and click the “Save Changes” button.
Now when you click the “Send” button on a message, you’ll see a message appear at the top of your inbox with an “Undo” link. As long as the message is on the screen, you can click the “Undo” link and the email will reappear, ready for editing or deleting.
link
http://www.foxnews.com/tech/2015/07/25/how-to-unsend-email/