Design an Email Service (SMTP/Sending)

To design an email service capable of sending billions of emails reliably, I would start by defining the core components: an ingestion API, a distributed message queue, a worker pool, and a reputation management system. First, incoming requests hit a load-balanced API layer that validates user input and pushes messages into a partitioned Kafka cluster. We partition by recipient domain to ensure that if one domain has issues, it doesn't throttle the entire system. Next, a scalable worker pool consumes these partitions. Crucially, workers must be stateless and idempotent; we use unique message IDs to prevent duplicate sends during network retries. For deliverability, we implement a sophisticated bounce handler. Instead of blocking, we listen to SMTP server responses asynchronously. Hard bounces are immediately flagged to suppress future sends, while soft bounces trigger an exponential backoff strategy before retrying. To manage spam filters, we maintain separate IP pools for different sender reputations and strictly enforce rate limiting based on historical data from each receiving domain. This prevents us from being blacklisted. Finally, we integrate real-time dashboards to monitor queue lag and delivery success rates, allowing us to dynamically adjust worker counts or throttle traffic if error spikes occur, ensuring the system remains robust under Meta's massive scale.