Building MailTest: An Open Source Email Deliverability Debugger

The Problem

[Story about client's 50K emails going to spam]

Why Existing Tools Fall Short

• Expensive ($100+/month)
• No explanations
• SaaS lock-in

What I Built

[Demo GIF/screenshots]

Technical Architecture

Backend

• NestJS + Fastify (why not Express)
• BullMQ for async jobs
• RS256 JWT (why not HS256)

Frontend

• Next.js 15 App Router
• Radix UI primitives
• Real-time updates with SSE

Infrastructure

• Docker Compose
• PostgreSQL 16
• Redis 7
• Paddle billing

Interesting Challenges

1. IMAP Polling: How to catch emails reliably
2. Scoring Algorithm: Weighted checks (SPF 15%, DKIM 20%, etc.)
3. PDF Generation: Server-side with Puppeteer

Code Snippets

[Show interesting parts]

What's Next

• More integrations
• Auto-fix features
• API monetization

Try It

https://mailtest.scuton.com

Discussion

What features would you want in a deliverability tool?