Email sandbox for developers

How is an email sandbox different from a fake SMTP server?

A fake SMTP server is the capture mechanism that accepts a message and never relays it, while an email sandbox is the product built around that mechanism: shared inboxes, rendering previews, spam scoring, and an API for assertions.

Every sandbox contains a fake SMTP server; not every fake SMTP server is a sandbox. If you only want the capture mechanism and a config snippet for your framework, the fake SMTP server guide covers that on its own.

What does an email sandbox catch that logs miss?

An email sandbox stores the full MIME message, so it catches rendering failures, broken links, double-encoded tokens, high spam scores, and wrong recipients, none of which show up in a log line that says the send succeeded. These are the things that break in production:

How do teams organize email sandboxes?

Teams give each environment, feature branch, or workstream its own sandbox with its own SMTP credentials, so staging, dev, and QA traffic stay separate while everyone in the organization can open any inbox. The useful pattern is one sandbox per workstream, not one giant inbox for everything:

• One per environment. A staging sandbox and a dev sandbox keep test traffic separate. QA reviews release emails in the staging inbox without digging through local development noise.
• One per feature branch. Working on a new onboarding flow? Create a sandbox for the branch, point the branch's config at it, delete it when the branch merges. On Mailloop, creating a sandbox is one click or one API call.
• Shared by the whole team. Everyone in the organization sees the same captured emails, so a designer can review the rendered template a backend developer triggered. Mailloop's Basic plan ($12/month) covers 5 members and 5 sandboxes; Pro ($29/month) covers 10 members and 20 sandboxes.
• Receiving external email too. Each Mailloop sandbox has a catch-all address at [email protected], so it also captures inbound mail from third-party services you integrate with.

How do I test emails in CI?

To test emails in CI, create a throwaway sandbox at the start of the test, point your app's SMTP config at it, trigger the action that sends email, block until the message arrives, and assert on its content. Visual inspection does not scale to a test suite. With @mailloop/sdk from npm:

import { MailloopClient } from '@mailloop/sdk';

const client = new MailloopClient({ apiKey: process.env.MAILLOOP_API_KEY });

// One throwaway inbox per test, deleted automatically
const sandbox = await client.sandboxes.createTemporary({ duration: 60 });

// Your app sends through sandbox.mailloop.io
// using sandbox.username / sandbox.password as SMTP credentials

await registerUser('[email protected]');

// Block until the email lands, then assert on it
const email = await client.emails.waitFor(sandbox.id, {
  subject: 'Welcome',
  timeout: 30000,
});

expect(email.to).toContain('[email protected]');
expect(email.html).toContain('Confirm your account');

Three details make this reliable in CI. Temporary sandboxes auto-delete after a set duration, so parallel test runs never share an inbox and nothing piles up. The waitFor call polls server-side, so there is no sleep-and-retry loop in your test code. And because capture is keyed on SMTP credentials, the test passes or fails on what your app actually sent, not on a mock.

For event-driven setups, sandboxes can also fire webhooks when an email arrives (Basic plan and up), which lets a separate job kick off the moment the message lands instead of polling at all.

Can an AI agent run the email test?

Yes: an agent connected to a sandbox over MCP can create a temporary inbox, run your signup flow, wait for the welcome email, read its content and links, and report what broke, all without a human opening an inbox. Mailloop runs a hosted MCP server at https://api.mailloop.io/mcp with OAuth, so Claude Code, Cursor, or claude.ai can do the whole loop with wait_for_email and get_email.

That turns "did the confirmation email go out with the right link?" into something you can ask your coding agent mid-session, without leaving the editor or pasting an API key. The agent skills that teach the workflow live at github.com/mailloop/skills. Setup instructions for each client are on the MCP server page.

Which email sandbox should you use?

Use a hosted sandbox such as Mailloop, Mailtrap, or Mailosaur when a team shares inboxes or CI runs in the cloud, and use self-hosted Mailpit when you only test on your own machine and want zero cost. The main options:

If you only test on your own machine and want zero cost, run Mailpit: a single Go binary, actively maintained, with a REST API and webhooks. Its trade-off is that it is local by design, so shared team inboxes and CI staging need extra networking, and spam scoring requires a separately running SpamAssassin server.

Mailosaur is strong when multi-language SDK support for Playwright or Cypress suites is the whole job, and it also tests SMS, but there is no permanent free tier. Mailtrap bundles a sandbox with a transactional sending platform, which fits if you want both from one vendor; its free sandbox caps at 50 emails per month. Mailloop's free plan captures 500 emails per month into 1 sandbox, and is the only one of the four with an official OAuth MCP server, so coding agents connect without an API key. Full comparison on the alternatives page, or read the ranked comparison of 8 email testing tools in 2026 for the wider field.