Oops, you accidentally sent a nasty e-mail to an ex- that you didn’t mean and you want to take it back. Good news if you are using GMail and run into this type of problem. If you send an e-mail that you immediately regret, GMail will allow you to unsend it. Here’s how it works:
Click on Settings and go to GMail Labs. Enable “Undo Send” by Yuzo F and Save the Settings. After that is enabled, when you send out messages, you will notice the Undo button by the confirmation message.
If you click Undo, GMail will take back the message and return you to the compose part of the message. The message is held for only about 5 seconds so you have to react quickly if you regret sending the e-mail.
[via GMail Blog]