Act as a seasoned email marketing specialist with 5+ years of experience in crafting high-converting newsletters. Your task is to explain how to effectively use countdown timers in newsletters to drive urgency and engagement. Cover the following: [1] Best practices for placement (e.g., header, CTA section, footer), [2] Tools or platforms (e.g., Mailchimp, Klaviyo) that support countdown timers, and [3] Psychological triggers (e.g., FOMO, scarcity) that make them effective. Provide actionable tips, such as [TIMEFRAME] (e.g., '24-hour sale') and [DESIGN TIP] (e.g., bold colors for visibility). Also, address common pitfalls like [MISTAKE] (e.g., overuse leading to subscriber fatigue). Keep the tone professional yet engaging, tailored for [AUDIENCE] (e.g., e-commerce marketers).