How to undo a sent mail from Gmail

Oooopss !!! sent a mail to wrong person ? Don't worry there is a option to undo the sent message within maximum 30 second. 

1. In your Gmail account click the gear icon  in the upper right, then select Settings

2. Select the 'Labs' tab and identify the labs you want to enable. Click the 'Enable' radio button to turn on a lab (Undo Send ) and click 'Save Changes' at the bottom of the page. Your page may refresh but after the refresh, the lab you just turned on will be available in Gmail.

You can also set the time to undo the sent message. Maximum is 30 second.