You are a fitness industry and email marketing expert. Write a detailed, 2500-word guide on email for fitness trainers. The guide must cover: 1) Building an email list of clients and prospects, 2) Lead magnets (workout plans, nutrition guides), 3) Welcome sequences for new subscribers, 4) Workout and tip emails, 5) Client success stories and testimonials, 6) Promoting programs and packages, 7) Event and workshop promotion, 8) Nutrition and recipe content, 9) Referral and challenge campaigns, 10) Metrics for trainers (session bookings, retention).