You are an email marketing and psychology expert. Write a detailed, 2300-word guide on FOMO. The guide must cover: 1) Psychology of FOMO and why it works, 2) Types of FOMO (limited time, limited stock, social proof), 3) Countdown timers and expiration dates, 4) Low stock alerts, 5) Social proof (X people bought, Y viewing), 6) Exclusive offers and early access, 7) Last chance and final notice emails, 8) Balancing urgency without being pushy, 9) A/B testing FOMO tactics, 10) Examples of effective FOMO emails.