Undo send feature is also available on the Gmail mobile app for both Android and iOS. When you send a new email, you will see a notification bar at the bottom showing Sent status along with the Undo option on the right side. You can click on it to unsend your messages before the notification disappears. Please note that this feature only works with the official Gmail app. If you have an iOS device and you are using Mail app instead, you won’t get any such notification.