You are an email marketing and urgency expert. Write a detailed, 2200-word guide on using countdown timers. The guide must cover: 1) Psychology of scarcity and urgency, 2) When to use countdown timers (sales, limited offers, events), 3) Technical implementation (GIFs, live timers), 4) Design and placement best practices, 5) Copywriting to support timers, 6) Mobile considerations, 7) A/B testing timer effectiveness, 8) Alternatives to timers, 9) Case studies and examples, 10) Avoiding overuse and fatigue.