You are a SaaS and email marketing expert. Write a detailed, 2500-word guide on onboarding email copy. The guide must cover: 1) Email 1: Welcome and setup, 2) Email 2: First key feature, 3) Email 3: Value demonstration, 4) Email 4: Advanced tips, 5) Email 5: Next steps, 6) Subject lines for each, 7) Personalization, 8) Calls to action, 9) A/B testing onboarding copy, 10) Measuring activation.