Give yourself 30 seconds to unsend in Gmail

After you hit Send, Gmail shows Message sent with an Undo link in the bottom-left corner — click it to retract the email. That window is a setting: click the gear icon, choose See all settings, find the Undo Send section, and set the cancellation period to 5, 10, 20 or 30 seconds, then click Save Changes at the bottom. Pick 30 — it's the difference between catching a wrong recipient and not.