Our recommendation, up front. We make Proxy Mail QA, so we’re not neutral, and it covers only one row of this toolkit: checking your sign-up, log-in and account emails. Speed, accessibility, SEO, broken links, security and how the site looks on different devices all need other tools, most of them free. We name them below; none of them are ours.
While you build
| Tool | What it catches |
|---|---|
| A type checker (TypeScript) | Wrong data in the wrong place, before you run anything |
| A linter (ESLint) and a formatter (Prettier) | Common mistakes and inconsistent code |
| Your browser’s DevTools | Errors in the console, failed requests in the Network tab, layout in device mode |
| Preview deployments (Vercel, Netlify, Cloudflare Pages) | A link for every change, so it’s checked on a real server before it’s merged |
| An AI coding agent (Qoder, Claude Code, Cursor) | A second read of each change, and running the checks below for you |
Before and after it goes live
| Area | What to check | Tools |
|---|---|---|
| Speed | Load time on a phone, layout shifts, image sizes | Lighthouse (in Chrome DevTools), PageSpeed Insights (which adds real visitors’ data once you have traffic), WebPageTest |
| Accessibility | Labels, contrast, headings, keyboard use, screen readers | axe DevTools, WAVE, Lighthouse; then the keyboard alone and a screen reader (VoiceOver, NVDA). Automated tools find only some problems |
| HTML and links | Invalid markup, broken links, missing pages | The W3C Markup Validation Service, the W3C Link Checker, or a crawler such as Screaming Frog |
| Devices and browsers | Layout on small screens, Safari and Firefox differences | DevTools device mode, a real phone, BrowserStack or LambdaTest for devices you don’t have |
| SEO | Pages indexed, titles and descriptions, structured data | Google Search Console, Bing Webmaster Tools, Lighthouse’s SEO audit, Google’s Rich Results Test |
| Security basics | HTTPS set up properly, security headers, known-vulnerable packages | Qualys SSL Labs, Security Headers, npm audit or Dependabot, ZAP (formerly OWASP ZAP) against your own staging site |
| Sign-up, log-in and account emails | The email arrives, its code or link works, it’s sent properly, exactly one arrives | Proxy Mail QA. For seeing emails during local development only, Mailpit |
| Whole flows | Sign-up, checkout and other journeys still work after a change | Playwright or Cypress |
| After launch | The site is up, errors are seen, people find what they need | An uptime monitor (UptimeRobot, Better Stack), Sentry, your analytics |
What Proxy Mail QA checks, and what it doesn’t
It checks the parts of your site that depend on an email:
- A new private test address for every try, so a test never picks up an old code.
- The verification code or sign-in, invite or reset link, pulled out of the email.
- How the email was sent: SPF, DKIM and DMARC, links to
localhostor plainhttp, where the link goes, whether it says when it expires, and a plain-text part. - That exactly one email arrived, or that none did.
- Unusual addresses, names and passwords to try in the form, so you can see how it handles them.
You can use it by hand with Proxy Mail QA for Chrome, in Playwright or Cypress through its npm package, or from an AI agent through its MCP server.
It doesn’t check design, wording, speed, accessibility, SEO or security, and it doesn’t receive SMS.
Let an agent run the checks
An AI agent such as Qoder’s can run most of this toolkit for you on a preview deployment: Lighthouse from the command line, a link checker, your Playwright tests, and, with Proxy Mail QA connected from the Qoder Marketplace, the sign-up email. A prompt to start from:
Check the preview at https://acme-git-new-pricing.vercel.app before I merge.
1. Run Lighthouse for the home page and the sign-up page, on mobile. List anything
under 90, with the biggest fixes first.
2. Run a link check over the site and list broken links.
3. Sign up with a new Proxy Mail QA address, finish the verification, and run
check_email with expect_host set to the preview's host. count_emails should be 1.
4. Give me one list of what to fix, most important first.
Still look at the site yourself, on a phone, before it goes live. No tool tells you whether it’s clear.
For a step-by-step review of a whole site, see Help me check my website.