Sending transactional emails remains a core requirement for web applications, especially for user authentication flows. This tutorial demonstrates how to build an email OTP verification system using Node.js, Convex as the backend, and the AutoSend API for email delivery. AutoSend, a developer-friendly email platform, provides high deliverability and REST API access without the complexity of domain setup.

While it lacks a free tier, its $1/month hobby plan offers 3,000 emails. Convex handles real-time data sync and serverless functions, managing OTP storage, user verification, and session tokens. The implementation uses Convex mutations for OTP creation and validation, ensuring secure one-time access.

A 30-second grace period prevents timing issues. On the frontend, a basic HTML form collects user email and OTP input. Express.js serves as the web server, routing requests between the client, Convex, and AutoSend.

Developers can extend this foundation for broader transactional email needs—order confirmations, password resets—while benefiting from AutoSend's analytics and compliance features.