How many times have you hoped that the e-mail that you just sent gets lost in the web so that the recipient doesn't see it? It might happen because the second you hit the send button, you realize there's a terrible misspelling or any kind of typo, perhaps you notice that you forgot to include an important detail or maybe that you could've rephrased what has been said in order to sound more polite or simply just better. Worst case scenario, the e-mail is being sent to a wrong recipient! Gmail has had an option to revert the sending process a few seconds after you hit send for several years now, but since it was hidden under the Gmail Labs, a section where users can try some experimental features, most users wouldn't know about it.
Now, Google is bringing the Undo Send functionality into the regular settings for Gmail so it can be accessed by a wider audience. What you need to do to activate this is to click on the gear icon in the top right side, go into Settings and in the General tab you will see this new option, if you enable it, you can choose how much time the yellow bar on top of the screen with the button to Undo Send will appear in case you end up changing your mind, 5, 10, 20 or 30 seconds are the options. In case users already have activated the option through Gmail Labs, it will be turned on by default, so they don't need to do this process unless they want to set a different amount of time for the button.
This feature was introduced along with another Gmail product called Inbox, which organizes emails in a whole new way as well as adding reminders and other useful features, but when it launched, only users with an invite could access to this new app. Right now, it is available for anyone who wants to try it, it's available in a browser version and as a mobile app, where the Undo Send feature has always been available, so perhaps this feature will make it to the Gmail's mobile app.