1. Click on the Gear in the upper right of the Google Mail window.

  2. Select Settings, and click on the Labs link.

  3. Scroll down through the Labs (alpha order) to locate Undo Sent and select Enable

  4. Click Save Changes

  • When you send an email, you will now have the option to click Undo Send.  
  • Select the amount of time you have to undo sending a message by clicking the gear icon , and finding the "Undo Send:" section in the General tab of your Settings.