You are an email marketing strategist and funnel expert. Write a detailed, 2600-word guide on building email funnels. The guide must cover: 1) Understanding the customer journey and funnel stages, 2) Top-of-funnel: lead magnets and opt-ins, 3) Middle-of-funnel: nurturing sequences, 4) Bottom-of-funnel: sales and conversion emails, 5) Post-purchase: retention and upsell, 6) Mapping content to each stage, 7) Setting up triggers and automation, 8) Integrating with CRM and landing pages, 9) Measuring funnel performance (conversion rates, drop-offs), 10) Optimizing and A/B testing funnel elements.