I hear that Gmail has ability to delay sending emails for up to 30 seconds so that you can undo the send, but I am unable to locate the setting.Where does this setting exist in Gmail?
How to delay Gmail send of messages to allow 'undo send' feature?
gmail
If you make a typo or regret sending a message, you can undo the  action by enabling Undo send. This setting gives you the option to  take back a message you just sent.To enable Undo Send:Click the gear in the top right Select Settings.Scroll down to Undo Send and click Enable.Set the cancellation period (the amount of time you have to decide if you want to unsend an email).Click Save Changes at the bottom of the page. If you had Undo Send turned on in Gmail Labs, your Undo Send setting will be on by  default.Note: To give you time to undo, Gmail delays sending the message for a few seconds. So if you don't select Undo within the time  limit, your message will be sent. Undo send may not work if you  experience connection issues.Taken from this Google Support article.