Email marketing success is measured by how people react to your emails. Open rate shows how many people open the message. Click rate shows how many click links inside. Replies and sales show stronger interest. Unsubscribe rate shows when content is not helpful. Comparing results between emails helps you learn what subject lines work better. Checking results after each campaign helps improve the next one. Numbers matter, but clear messages and timing matter too. Which email numbers should businesses pay attention to most in email marketing today?