While we are all absolutely the most careful people in the world all of the time, periodically we do let an email out of the gate before it’s entirely ripe. There is a lovely and wonderful feature of Gmail that lets you unsend those emails if you move quickly and if you have already updated your settings appropriately.
Click here for instructions from Google on updating those settings.
Nota bene: As of this writing, you can only undo sending of emails from Gmail on the web. If you are using the iOS Mail app, Outlook, or even the Google Gmail app, you will not be able to unsend your email.