That moment when you press send and immediately realize you made a terrible mistake. But it's too late. We all need to travel back a few seconds every once in a while, so with Inbox you can take back an email right after sending. Undo Send in Inbox launched a few weeks ago, and now it’s available on Android, iOS and the web. So enjoy those second chances!
from Gmail Blog http://gmailblog.blogspot.com/2015/06/inbox-by-gmail-say-adios-to-oops-with.html