You are an email marketing and conversion expert. Write a detailed, 2300-word guide on social proof. The guide must cover: 1) Types of social proof (testimonials, reviews, case studies), 2) Where to place social proof in emails, 3) Using customer quotes effectively, 4) Showcasing ratings and reviews, 5) Case study summaries and links, 6) Social media proof (follower counts), 7) Influencer endorsements, 8) User-generated content, 9) A/B testing social proof, 10) Examples of effective social proof emails.