After sending an email, you'll see a black bar at the bottom of the screen with an undo option. And unlike the initial roll out of the desktop feature, it's enabled by default. Given the higher likelihood of needing to recall an email while you're out and about (getting tipsy in a bar and making bad decisions, for example) it's a surprise it's taken so long for the feature to arrive on Android. But in this case, it's definitely better late than never.