Build an Email Deliverability Checker with Claude Code

Build an Email Deliverability Checker web app. People search for "email deliverability checker" about 700 times a month, plus related searches like "email spam checker", "email deliverability test", "spam score checker". The goal is a tool that ranks for those searches and makes money.

What it should do:
Create an email deliverability checker with email content input and domain reputation analysis. Test spam score, authentication (SPF, DKIM, DMARC), and inbox placement. Include spam filter testing across major providers and improvement recommendations. Add email preview and authentication setup guide. Focus on getting spam score analysis working first.

How to build it:
1. Read this folder first. If it is empty, set up Next.js (App Router, TypeScript, Tailwind). Write a short PLAN.md with the pages, components, any library or API you need, and what you are leaving for later. Then build.
2. Put the tool itself on the homepage so a visitor can use it within seconds, with no sign-up. Do the processing in the browser where you can, so it is fast and cheap to run. Look at how Mail Tester, SendGrid, Mailgun do it and make ours simpler and faster.
3. Make it rank. Use "email deliverability checker" in the title tag, H1 and meta description. Under the tool, add a short how-to and an FAQ with FAQPage schema, and add a sitemap and robots.txt. Scaffold these programmatic pages from one template and a data file: /email-deliverability-[industry], /spam-checker-[email-type], /inbox-test-[platform].
4. Monetisation: Freemium: 10 tests/month, $12/mo unlimited + detailed reports. Add a pricing page and the free usage limit, and leave Stripe checkout as a clearly marked stub with the environment variables it needs.
5. Run it locally and test the main flow end to end, including edge cases like empty input and very large files. Fix what breaks, then finish with a summary of what works, what is stubbed and how to deploy it to Vercel.

Paste it into Claude Code in an empty folder. It plans first, so read PLAN.md and steer before it starts building.