Most of us always fall victim of such situation when you have already clicked the SEND button after some second you remember to attach a file to the e-mail, or some common situation when you sent an angry email to a friend and you wish you can take it back, Well here is your chance.
The Undo Send feature is now an official part of Gmail, no longer relegated to mere beta status in Gmail Labs. In honor of its new status, let's see how the the feature works
source : http://www.codingsavvy.com/2015/06/trick-to-unsend-sent-email-from-your.html