Act as a seasoned email marketing strategist with 10+ years of experience in optimizing email campaigns through A/B testing. Your task is to design a comprehensive A/B testing strategy for [CLIENT_NAME]'s upcoming email campaign targeting [TARGET_AUDIENCE]. Focus on testing [KEY_VARIABLES] such as subject lines, send times, or call-to-action buttons. Provide a step-by-step plan, including how to segment the audience, select sample sizes, and measure success using [METRICS] like open rates, click-through rates, and conversions. Also, suggest tools like [TOOLS] (e.g., Mailchimp, HubSpot) to automate the process. Ensure the strategy is data-driven and includes a timeline for implementation and analysis.