---
title: "Test your email integration with sandbox mode"
description: "Send real API and SMTP requests to a sandbox project. Lettermint handles every email as usual, except nothing gets delivered."
url: "https://lettermint.co/changelog/test-your-email-integration-with-sandbox-mode"
published: "2026-10-08"
last_updated: "2026-10-08"
---

# Test your email integration with sandbox mode

> Send real API and SMTP requests to a sandbox project. Lettermint handles every email as usual, except nothing gets delivered.

Testing an email integration usually means sending a few emails to
yourself or to a throwaway address. That's fine once or twice. Do it
every time you change something, and the bounces and repeat sends can
start to hurt your domain's reputation.

Sandbox mode takes care of that. You keep sending your
[transactional emails](https://lettermint.co/features/transactional-emails) through the API
or SMTP exactly as before, and nothing ever lands in an inbox.

## What is sandbox mode?

Sandbox mode is a per-project setting. Lettermint accepts and
processes each email like it normally would, and stops right before
handing it to the recipient's mail server. Validation still runs, so
an invalid address gets the same error you'd see in production. Every
email shows up in your dashboard.

Sandbox emails **don't count toward your monthly usage**. They're left
out of your statistics, and a simulated hard bounce never ends up on
your suppression list.

![Lettermint Sandbox](https://lettermint.co/content/changelog/lettermint-sandbox.webp)

## How it works

### Turn on sandbox mode

Pick **Sandbox** as the delivery mode when you create a project. For
an existing project, flip the toggle under **Delivery mode** in
**Settings**. Your API tokens and SMTP credentials stay the same, so
your code doesn't need to change.

### Simulate a result

By default, every sandbox email is marked as delivered. To see how
your app deals with a hard bounce or a spam complaint, set
`sandbox_result` on the API request. Sending over SMTP? Use the
`X-Lettermint-Sandbox-Result` header instead.

```bash
curl -X POST "https://api.lettermint.co/v1/send" \
  -H "Content-Type: application/json" \
  -H "x-lettermint-token: lm_project_token" \
  -d '{
    "from": "sender@acme.com",
    "to": ["recipient@acme.com"],
    "subject": "Welcome to Acme",
    "text": "Thanks for signing up.",
    "sandbox_result": "hard_bounced"
  }'
```

### Webhooks

[Webhooks](https://lettermint.co/changelog/webhooks-for-team-projects-or-routes) keep
firing in sandbox mode, so you can test your handlers end to end. One
thing to watch: new webhooks default to **Live**, which means they
skip sandbox events. Set them to **Sandbox** or **Both** to receive
them while testing.

## Going live

When you're done testing, switch the project to **Live** and emails
start going to real recipients. Make sure your domain is verified
first. Live mode won't send from an unverified domain.

## Available on Starter, Growth and Pro

Sandbox mode is included in the [Starter, Growth and Pro](https://lettermint.co/pricing)
plans. You can turn it on for any project in your
[dashboard](https://app.lettermint.co). The
[sandbox mode docs](https://lettermint.co/docs/platform/projects-and-routes/sandbox-mode)
list every result you can simulate and explain how to tell sandbox
events apart from live ones.