The Undo Send feature in Gmail allows users to recall emails sent within a set timeframe, which can be extended from the default five seconds to up to 30 seconds. To unsend an email in Gmail from a desktop, a user simply needs to click "Undo" in the popup that appears after sending the email, and then edit the message before resending it. The process is similar for using the mobile app. Users can also change the timeframe allowed for recalling emails by going to settings. However, deleting an email or clicking "View message" will prevent users from unsendings it.