---
title: "Test sign-up emails in apps built with Cursor · Proxy Mail QA"
description: "Cursor's agent can write your sign-up, magic-link and password-reset code. Here's how to check the real emails after each change, with a private test inbox, plus prompts and a project rule that prevent the usual bugs."
url: https://proxymail.qa/built-with/cursor
---

1. [Home](https://proxymail.qa/)
2. [AI app builders](https://proxymail.qa/built-with)

# Testing auth emails in apps you build with Cursor

Cursor's agent can implement sign-up, login and password reset across your codebase in one go. Whichever auth service you use, the emails only prove themselves when a real one lands in a real inbox, so check them after every change that touches auth.

Updated October 12, 2026

## Why the email path needs a real check

Cursor’s agent is good at the code around authentication: forms, routes, token handling and email templates. But a change can pass every test and still send an email that links to `localhost`, never arrives because the sender isn’t set up, or contains a code that expires before anyone could type it. Those show up only when a real email reaches a real inbox.

## Check it after every auth change

1. **Run the app** locally, or open the preview deployment for your branch.
2. **Create an address** in Proxy Mail QA, one per scenario: new user, invited user, password reset.
3. **Go through the flow** the change touched, using the code or link from the address’s row.
4. **Read the whole email once** in the inbox view: sender, subject, wording and every link.
5. **Note what you found** on the address, so it stays with the test account.

Links to `localhost` on a preview or staging deployment are flagged on the row. [Here’s how to fix them](https://proxymail.qa/solve/localhost-links) in Supabase, Firebase, Auth.js and other stacks.

## Copy-paste prompts for Cursor

```text
List every email this app sends: sign-up confirmation, magic link, password reset, invite and email change. For each, show where it's triggered in the code, which service sends it, how its link URL is built, and how long its code or link is valid. Flag hard-coded localhost URLs.

```

```text
Build every link in auth emails from an environment variable for the app's public URL, with localhost only as the development default. Add the variable to .env.example and the deployment settings.

```

```text
The email says the code is valid for 10 minutes. Make the server enforce the same expiry, and make requesting a new code invalidate the old one.

```

## A project rule that prevents the usual bugs

Save this as a project rule (for example `.cursor/rules/auth-emails.mdc`) so the agent follows it whenever it touches auth:

```text
When changing sign-up, login, password reset, invites or any email-sending code:
- Build links from the app's public URL setting, never a hard-coded localhost.
- Keep the expiry stated in an email equal to the one the server enforces.
- Make single-use links and codes stop working once used or replaced.
- Tell me which emails to test by hand after the change.

```

## Try it on your next sign-up test

Free plan, no credit card. Create an address and watch the code or link arrive.

[Create a free account](https://app.proxymail.qa/?mode=signup)

## Questions

### Which auth services does this work with?

Any that send email: Supabase Auth, Firebase Authentication, Clerk, Auth0, Auth.js with an email provider, or your own code sending through Resend, Postmark or SendGrid. Proxy Mail QA receives the email like any inbox.

### Can Cursor's agent read the code from the email itself?

Yes. Add the Proxy Mail QA MCP server to .cursor/mcp.json with an API key, and the agent can create an address and wait for its code or link itself. See proxymail.qa/docs/api for the snippet.

### Do I need to change my app to test with Proxy Mail QA?

No. Sign up with a Proxy Mail QA address like any user would.

## Related guides

- [Claude CodeCheck the auth emails in apps you build with Anthropic’s coding agent, from localhost to staging.](https://proxymail.qa/built-with/claude-code)
- [OpenAI CodexRun the real email flow once before you merge a Codex change that touches sign-up or login.](https://proxymail.qa/built-with/codex)
- [ReplitMake sure sign-up and reset emails link to your published Replit app, not the dev URL.](https://proxymail.qa/built-with/replit)
- [LovableFix the classic Supabase localhost redirect and check every auth email before you launch.](https://proxymail.qa/built-with/lovable)
- [Base44Test sign-up, invite and login emails, and every user role, with a test account per role.](https://proxymail.qa/built-with/base44)
- [BoltTurn on email confirmation, fix the localhost Site URL and test every auth email before you publish.](https://proxymail.qa/built-with/bolt)

[See every email testing problem we solve →](https://proxymail.qa/solve)

Cursor is a product of Anysphere. Proxy Mail QA is independent and isn't affiliated with or endorsed by them.