testing-with-sandbox

---
name: testing-with-sandbox
description: >-
  Use when capturing outbound email in development or staging without
  delivering to real recipients, inspecting HTML or headers, running spam or
  structure checks, or automating tests against a fake inbox. Use when testing
  outgoing mail from an app without committing to a production ESP yet. Use when
  using Mailtrap Email Sandbox, Sandbox API, or sandbox-mode sending.
---

# Testing with Mailtrap Email Sandbox

## Overview

**Email Sandbox** captures mail in **sandboxes (test inboxes)**—a test environment where messages are **not** delivered to real recipients. You can send to sandboxes using our **SDKs**, **HTTP API**, or **SMTP**, depending on your needs.

**Before generating SDK code:** read the README of the relevant SDK repository (see `sending-emails`) for current sandbox mode options, **inbox id**, and constructor flags. Do not rely on memory.

**Related skills:** `authorizing-api-requests` (sandbox token scope, env vars, `account_id` resolution), `sending-emails` (live sending hosts and streams), `using-email-templates` (preview template sends in sandbox).

## When to use

- You want **no real delivery**: dev, staging, CI, or demos where mail must stay in a **test inbox**.
- You need to **inspect** what was sent: bodies, headers, attachments, or basic checks (e.g. spam report) via **Sandbox / Testing API** or the **UI**.
- You are **automating** tests against captured mail.
- You will **only change SMTP settings** so an existing app sends into a sandbox—no need for a framework-by-framework tutorial from this skill.

## When not to use

- **Live** sends to real recipients (`sending-emails`).
- For full framework setup guides or detailed API references, link users to Mailtrap's Integration tab for SMTP/API details and the [API docs](https://docs.mailtrap.io/developers/) for specifics—don't cover every framework or API field here.

## Quick reference

### API base

| Service                  | Send mail URL                                         | Auth header examples                              |
| ------------------------ | ----------------------------------------------------- | ------------------------------------------------- |
| Email Testing API (REST) | `https://sandbox.api.mailtrap.io/api/send/{inbox_id}` | `Authorization: Bearer $MAILTRAP_SANDBOX_API_TOKEN` |

### Tokens and account_id

Sandbox uses a **separate** token (`$MAILTRAP_SANDBOX_API_TOKEN`, Testing/Sandbox scope) — never reuse the live `$MAILTRAP_API_TOKEN`. The `account_id` in the example endpoints below is resolved at runtime via `GET https://mailtrap.io/api/accounts`. Full token scope, storage, and `account_id` resolution: see skill `authorizing-api-requests`.

### When to use API vs SMTP

Use **SMTP** when testing apps that already send mail via SMTP (just update the host, port, and credentials).
Use the **HTTP API** when building new integrations or your app can make HTTP requests; it's better for programmatic testing and automation.

### SMTP settings (sandbox)

| Setting             | Value                                                                       |
| ------------------- | --------------------------------------------------------------------------- |
| Host                | `sandbox.smtp.mailtrap.io`                                                  |
| Ports               | 2525 (default), 25, 465 (SSL), 587                                          |
| Username / Password | Per **sandbox** credentials from the **Integration** tab in the Mailtrap UI |

**Never use sandbox credentials or endpoints in production. Messages will only be captured in the sandbox, not delivered.**

### Key parameters

- **Inbox ID**: Every sandbox (test inbox) has a unique **inbox id**, visible in the UI URL and needed for sending or REST API operations.
- **Token scope**: Use a token with permissions for the relevant project and test inbox.

### Typical use cases

- Capture all outbound mail in dev, test, or staging (no real recipients).
- View, validate, and assert message headers, bodies, HTML, attachments, or spam score.
- Run integration or CI checks that read from the Email Sandbox API.
- Test Mailtrap **templates** by pointing API or SDK/SMTP at `sandbox.api.mailtrap.io` / `sandbox.smtp.mailtrap.io` with a valid inbox id.

### Example API paths

Use [API docs](https://docs.mailtrap.io/developers/) for details, but typical endpoints include:

| Operation       | URL                                                                                          | Reference                                                                                 |
| --------------- | -------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------- |
| List sandboxes  | `GET https://mailtrap.io/api/accounts/$MAILTRAP_ACCOUNT_ID/inboxes`                          | [Sandboxes API](https://docs.mailtrap.io/developers/email-sandbox/sandboxes-inboxes.md)   |
| List messages   | `GET https://mailtrap.io/api/accounts/$MAILTRAP_ACCOUNT_ID/inboxes/{inbox_id}/messages`      | [Messages](https://docs.mailtrap.io/developers/email-sandbox/messages.md)                 |
| Fetch a message | `GET https://mailtrap.io/api/accounts/$MAILTRAP_ACCOUNT_ID/inboxes/{inbox_id}/messages/{id}` | [Message details](https://docs.mailtrap.io/developers/email-sandbox/messages.md)          |
| Send test email | `POST https://mailtrap.io/api/accounts/$MAILTRAP_ACCOUNT_ID/inboxes/{inbox_id}/messages`     | [Send test emails](https://docs.mailtrap.io/developers/email-sandbox/send-test-emails.md) |

For **template testing**, see the Integration tab of your template and [Handlebars](https://docs.mailtrap.io/email-api-smtp/email-templates/handlebars.md).

### SDKs

Official Mailtrap SDKs support sandbox/inbox operations and provide flags or methods to set **test mode** and **inbox id**. This allows you to use the same integration for both live sending and sandbox testing—simply change the mode or credentials depending on your environment (development, staging, or production). For install commands and language coverage, see [Mailtrap developer documentation](https://docs.mailtrap.io/developers/). Repository READMEs have the latest sandbox options:

- [Node.js](https://github.com/mailtrap/mailtrap-nodejs)
- [Python](https://github.com/mailtrap/mailtrap-python)
- [PHP](https://github.com/mailtrap/mailtrap-php)
- [Ruby](https://github.com/mailtrap/mailtrap-ruby)
- [Java](https://github.com/mailtrap/mailtrap-java)
- [.NET](https://github.com/mailtrap/mailtrap-dotnet)
- [CLI](https://github.com/mailtrap/mailtrap-cli)

### Common mistakes

| Mistake                                    | Fix/Explanation                                                                                          |
| ------------------------------------------ | -------------------------------------------------------------------------------------------------------- |
| Expecting real delivery from sandbox       | Mail in the sandbox is **never** delivered to recipients                                                 |
| Using production API token for sandbox     | Use a token with proper **sandbox/testing** scope, granting access to the target inbox                   |
| Forgetting **inbox id** parameter          | Always supply the **inbox id** (from UI or Integration tab) to associate messages with the correct inbox |
| Mixing sandbox and transactional endpoints | Testing API (`sandbox.api.mailtrap.io`) is **not** the same as `send.api.mailtrap.io` (live sending)!    |

### Sandbox email address

Each sandbox (test inbox) has an address like `alias@inbox.mailtrap.io` for inbound tests; plus-addressing can help isolate scenarios. See [Email address per sandbox](https://docs.mailtrap.io/email-sandbox/setup/email-address-per-sandbox.md) for limits and behavior.