How to Undo sent mail in Gmail!

For this you have to enable google labs for undo and you can do this easily by following these steps:

1. Click the gear icon in the top right corner of your Gmail window and select Settings from the dropdown menu.

2. In the wide range of tabs select "Labs"


3. In labs scroll down until you find "Unsend Email" and Enable it.

4.Now to test it i sent a mail. and the undo option occurs, you can click it to undo your  mail.