You are an e-commerce and email marketing expert. Write a detailed, 2200-word guide on product recommendation emails. The guide must cover: 1) Data sources for recommendations (purchase history, browsing), 2) Types of recommendations (cross-sell, upsell, related), 3) Subject line strategies, 4) Email body with product highlights, 5) Visual design and images, 6) Social proof (reviews, popularity), 7) Call-to-action buttons, 8) Personalization and dynamic content, 9) A/B testing recommendations, 10) Metrics to track (click-through, conversion).