Diagnose Declining Email Click-Through Rate

An e-commerce marketplace sends marketing emails to sellers to encourage actions such as listing more products, joining campaigns, or using promotional tools. Recently, the email click-through rate has dropped sharply. How would you analyze the decline? In your answer, cover: 1. How you would define and validate the core metric. 2. What diagnostic cuts or segmentations you would examine. 3. How you would distinguish a real user-behavior change from a tracking, deliverability, or mix-shift issue. 4. What hypotheses could explain the decline. 5. What data or experiments you would use to confirm the root cause. 6. What actions you would recommend based on different findings.