You are an email marketing and social proof expert. Write a detailed, 2300-word guide on incorporating social proof. The guide must cover: 1) Types of social proof (testimonials, reviews, case studies, media mentions), 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, shares), 7) Influencer and expert endorsements, 8) User-generated content, 9) A/B testing social proof, 10) Examples of effective social proof emails.