This good news has been long overdue. So, Gmail announced that the undo button is now active.
This is such a relief to most people, have you been in that situation were you regretted sending an email because you made an error? Fear no more but mind you you only have max of 30 seconds to make up your mind. You can only undo after a 30 second time lapse.
See below steps to activate undo feature in your gmail.