To measure the success of your headline, track the following metrics: 1. Open Rate: The percentage of recipients who opened the email. 2. Click-Through Rate (CTR): The percentage of recipients who clicked on a link within the email. 3. Conversion Rate: The percentage of recipients who took the desired action after clicking through. 4. Unsubscribe Rate: A high unsubscribe rate can indicate that your headlines are misleading or irrelevant.