Context An application sends transactional emails synchronously inside request handlers, causing slow API response times and dropped emails when the mail server is slow. The Problem Email sending blocks the HTTP response cycle and has no retry mechanism for failed deliveries. Constraints Use Bull to create an email queue backed by Redis. Add email jobs to the queue inside the request handler and respond immediately. Implement a worker that processes jobs with retry on failure (3 retries, exponential backoff). Handle failed jobs by logging to a dead-letter log. Expected Deliverable A Node.js application offloading email sending to a Bull queue, with a background worker, retry logic, and dead-letter handling.