What is testing-with-sandbox?

A guide for using Mailtrap Email Sandbox, a test inbox that captures outgoing email without delivering it to real recipients.

Use it to send mail through a sandbox, inspect message content, headers, and attachments, run basic checks, and test against a fake inbox.

It lets developers test email safely in development, staging, demos, or automated tests without accidentally emailing customers.

testing-with-sandbox — Skill for Claude Code and Codex by mailtrap

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add mailtrap/mailtrap-skills --skill testing-with-sandbox

Clone the repo

git clone --depth 1 https://github.com/mailtrap/mailtrap-skills

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/mailtrap/mailtrap-skills/testing-with-sandbox.svg)](https://agentmods.dev/skills/mailtrap/mailtrap-skills/testing-with-sandbox)

Your own site

<a href="https://agentmods.dev/skills/mailtrap/mailtrap-skills/testing-with-sandbox"><img src="https://agentmods.dev/badge/skills/mailtrap/mailtrap-skills/testing-with-sandbox.svg" alt="Measured on agentmods" height="20"></a>

Per session 72 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,610 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00072	$0.01610
Opus 5	$0.00036	$0.00805
Sonnet 5	$0.00014	$0.00322
Haiku 4.5	$0.00007	$0.00161

Measured 8d ago against content hash 4edde4177654, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-07, from the pricing page.

Security

Grade A, and why

testing-with-sandbox scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 8d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/testing-with-sandbox/SKILL.md · 109 lines

How it starts

The opening of the file, as written. The whole thing — 109 lines — stays where its author put it; the contents beside it link to each section on GitHub.

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 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.

Read the full file on GitHub · 109 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

8d ago First seen · 109 lines · 72 tokens per session scan A 4edde4177654

Subscribe to this mod's changes

testing-with-sandbox is a skill published in the GitHub repository mailtrap/mailtrap-skills (10 stars, last pushed 1mo ago), licensed MIT. It adds 72 tokens to every session and 1,610 once invoked, about $0.0004 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

Testing

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testing-with-sandbox