follow these steps:-
1. Click the gear icon in the top right corner of your Gmail window and select Settings from the dropdown menu.
2. Select Labs from the row of tabs.
3. Scroll all the way to the bottom where you see Undo Send and click Enable.
4. Hit Save Changes at the bottom.
5. Now when you send an email, the yellow dialogue that displays "Your message has been sent" will also, for a few seconds (the time is configurable) give you the option to Undo. The email is only actually sent once the Undo option disappears.
6. Just by clicking on that button you can undo your message.
Source: My answer to Facts and Trivia: What are the most useful facts you know?
1. Click the gear icon in the top right corner of your Gmail window and select Settings from the dropdown menu.
2. Select Labs from the row of tabs.
3. Scroll all the way to the bottom where you see Undo Send and click Enable.
4. Hit Save Changes at the bottom.
5. Now when you send an email, the yellow dialogue that displays "Your message has been sent" will also, for a few seconds (the time is configurable) give you the option to Undo. The email is only actually sent once the Undo option disappears.
6. Just by clicking on that button you can undo your message.
Source: My answer to Facts and Trivia: What are the most useful facts you know?