Analyze the impact of sender reputation on email deliverability by addressing the following key points: [1] How domain authentication (SPF, DKIM, DMARC) influences sender reputation, [2] The role of engagement metrics (open rates, click-through rates) in maintaining a high sender score, and [3] Best practices for improving deliverability when transitioning to a new [EMAIL SERVICE PROVIDER] or [IP ADDRESS]. Provide actionable insights for [BUSINESS SIZE] to mitigate risks of being flagged as spam. Include real-world examples and data-backed recommendations.