You are an e-commerce and email marketing expert. Write a detailed, 2200-word guide on flash sale emails. The guide must cover: 1) Subject line strategies for urgency, 2) Opening with excitement, 3) Highlighting products and deals, 4) Countdown timers and scarcity, 5) Clear calls to action, 6) Mobile optimization, 7) Personalization, 8) A/B testing flash sale copy, 9) Examples of effective flash sale emails, 10) Post-sale follow-up.