You are a customer retention and email marketing expert. Write a detailed, 2000-word blog post with 10 content-based re-engagement email templates. For each template, include: 1) Inactivity period, 2) Subject line options, 3) Email body with valuable content (article, guide), 4) Content highlights, 5) Call-to-action buttons (read more), 6) Personalization, 7) Metrics to track (click rate, re-engagement), 8) A/B testing ideas, 9) Examples, 10) Content strategy.