Google gMail Lab has a unique feature using which you can undo the sent mail. Time limit is 30 seconds.
The Labs feature name is “undo send” that lets you “unsend” a message immediately after sending it. If you accidentally clicked on the “Send” button or you realize that you forgot to attach an important file, you have a few seconds to click on “undo” and continue to edit the message.
undo send gMail Lab can be activayed from Gmail Lab page.
After enabling the undo send, you will get option to undo sent mail just after you send any mail.