When you enable Undo Send, you’re able to “unsend” an email up to ten seconds after you hit the send button — Gmail just delays sending briefly so you have a moment in case you change your mind.
So how do you enable Gmail’s undo feature?
Go to the little cog icon in the upper righthand corner and select “Settings.” About a third of the way down the page you’ll see the “Undo Send” section. You can choose between 5, 10, 20 and 30 second windows of unsendability. Make sure you hit “Save Changes” at the bottom and you’re all set.
Then, the next time you hit send on an email that you probably shouldn’t have, you’ve got precisely that many seconds to change your mind and hit the undo button at the top of the page.