Act as a seasoned email marketing strategist with 10+ years of experience in crafting milestone campaigns for SaaS companies. Design a step-by-step guide to creating a successful milestone email campaign that drives engagement and conversions. Include: 1. [TARGET_AUDIENCE] segmentation (e.g., new users, long-term customers, inactive subscribers) 2. [MILESTONE_TYPE] selection (e.g., 1-year anniversary, 100th login, feature adoption) 3. [CALL_TO_ACTION] optimization (e.g., limited-time offer, referral incentive, feedback request) Detail the ideal email structure (subject line, preheader, body copy, visuals) and A/B testing strategies for open rates. Provide 3 real-world examples of high-performing milestone emails, analyzing their psychological triggers (e.g., exclusivity, achievement, nostalgia).