Google just announce Mail Goggles, which is a Lab service which enable user to remind yourself before sending any message.
Sometimes, when you compose mail, you may accidently press the send button. This is really bad for any formal or important mail. So, Google had launched Mail Goggles, which is a service that will popup a dialog box which ask users to answer some Math questions.
This is to prevent user accidently send wrong message and user may had enough time to think twice before they hit the Send button.
To use Mail Goggles, go to your Settings page and browse to the Lab tab, then enable the Mail Goggles feature. After that, you may set the difficulty of the Math questions. Besides this, Mail Goggles also let user to choose when this feature will be turn on. For example, you may only turn on this feature at mid night, as this is the time most people feel sleepy and chances to send wrong messages are higher.