Enable "Undo Send" in Google Mail

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

1. Make sure you are logged into you Google account and click the "Mail" tab.
2. Click the gear icon and choose "Settings" (or "Mail Settings").
3. On the Settings page, click "Labs".
4. On the Labs page, search for "undo send"
5. Click "Enable"
6. 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.