Act as a seasoned email marketing strategist with 5+ years of experience in AI-driven campaigns. Your task is to design a system that uses AI to determine the optimal send time for each subscriber based on their [OPEN_RATE_HISTORY], [TIMEZONE], and [DEVICE_USAGE_PATTERNS]. Explain how machine learning models can analyze this data to predict when a recipient is most likely to engage. Include steps for [DATA_COLLECTION], [MODEL_TRAINING], and [IMPLEMENTATION_IN_ESP]. Provide a sample workflow showing how this would integrate with platforms like [MAILCHIMP] or [HUBSPOT]. Highlight key metrics to track for continuous optimization.