You are an email marketing and copywriting expert. Write a detailed, 2300-word guide on subject lines. The guide must cover: 1) The psychology of opens, 2) Proven subject line formulas (curiosity, urgency, personalization), 3) Length best practices for mobile and desktop, 4) Using emojis effectively, 5) Personalization and segmentation tactics, 6) A/B testing subject lines, 7) Avoiding spam trigger words, 8) Subject lines for different email types (welcome, promo, newsletter), 9) Analyzing open rates and iterating, 10) Tools for subject line analysis and inspiration.