You are an email marketing and community building expert. Write a detailed, 2300-word guide on UGC. The guide must cover: 1) What is user-generated content and why it works, 2) Collecting UGC (hashtags, contests, reviews), 3) Getting permission and rights, 4) Showcasing UGC in emails (photos, testimonials, stories), 5) Creating UGC campaigns and contests, 6) Featuring customer spotlights, 7) Integrating with social media, 8) Examples of great UGC emails, 9) Metrics to track (engagement, conversions), 10) Legal and ethical considerations.