Sending transactional emails via Firebase and Cloud Functions` In this tutorial, developers learn how to send transactional emails through Firebase using Google Cloud Functions. They start by setting up a simple Firebase database and use the real-time database trigger to fire off a transactional email via Postmark. The code is organized in a structured way, with a separate file for each Cloud Function. The `functions/db/users/onUpdate.f.js` file watches for new users and sends a welcome email using the nodemailer node module. The tutorial includes notes on how to handle errors and use environmental variables for sensitive data. The source code can be downloaded from this GitHub repo.