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?
It is in the 'Labs' section of settings, you have to enable this experimental feature. Search for 'undo send' to find it. You can then configure the delay of send from 5 up to 30 seconds in the General tab of your settings. It defaults to 10 seconds.