You can undo the email you just sent it if they did not read that mail. For enable this service ;
In the GMail page, in the right-top, click on settings
in the tab menu (which the first tab is General and the last one is Themes) click on Labs (2 tabs before the last tab)
Scroll dawn until you find Undo Send bye Yuzo F, enable it and save it..
now when you send an email, there is an option which you can undo the sent email.
Sending has been undone.
Any problems, just contact me