Gmail Labs provides an interesting feature to prevent you from sending mails that may you may regret later. It does so by asking you to confirm whether you really want to send that email.
So if you’re writing a mail late on a Friday night, when you may not be as sober as you would like yourself to be, you can get Gmail to ask you a few math problems before the message leave your Gmail outbox.
You can enable this via the “Labs” tab on the Gmail “Settings” page. By default, the Mail Goggles feature of Gmail become active late night on the weekend, but you can set it to other times as well from Settings – > General.