Transactional emails are automated emails triggered by user actions on web or mobile applications, requiring advanced logic like honoring user notification preferences and batching frequent messages. Courier's notification infrastructure helps pair email providers with transactional email features, making it easy to build production-ready systems and expand, monitor, and scale over time. This tutorial uses Twilio SendGrid via Courier's API to send transactional emails with Node.js, starting by integrating SendGrid with Courier, designing a transactional email template using the drag-and-drop editor, testing and verifying the email design, sending the email from an application using the provided code snippet, and debugging any issues that may arise.