Undo Send

Oops, hit "Send" too soon? Stop messages from being sent for a few seconds after hitting the send button.

Here’s one to help avoid all those “Oops!” moments. Google, being the awesome bunch of coders that they are, allow you to call back emails which you have just sent.
Steps to Undo the Message:

1) Go to Settings
2) Labs
3) Enable "Undo Send" functionality.
and done... :)

Next time you send an email accidentally, just call it back by clicking on Undo.