Google Mail doesn't have an "unsend" feature built in.
However, by using a "Labs" gadget, you can stop a message from being sent for a few seconds after clicking the "Send" button.
Enable the "Undo Send" Lab
- Make sure you are logged into you Google account and click the "Mail" tab.
- Click the gear icon and choose "Settings" (or "Mail Settings").
- On the Settings page, click "Labs".
- On the Labs page, search for "undo send"
- Click "Enable"
- Click "Save Changes"
From now on, when you send a message, you'll see a yellow message above the message list, with an "Undo" link. Click "Undo" to prevent the message from being sent.