Gmail labs introduced a new feature in Gmail, that is Undo send. This will help you to undo send a message. A link will come with the message sent status message, and you will have to click on that Undo link before 5 seconds, otherwise the mail will go. This is a very helpful feature, since i wished many times for this feature

Go to Settings=>Labs in Gmail and enable

Undo Send by Yuzo F if you need this feature