MailSandbox — email testing & simulation for developers

Fork of Mailpit adding Postmark API emulation and an MCP server for AI-assisted debugging.
Inspired by and grateful to the original work on Mailpit and MailHog. See Credits.

Repository • Installation • Usage • Postmark API • MCP Server

MailSandbox is a small, fast, low-memory, zero-dependency, multi-platform email testing tool & API for developers.
It provides an SMTP server, modern web interface, REST API, Postmark API emulation, and an MCP server for AI-powered workflows.

Features

🚀 Lightweight — single static binary or multi-arch Docker image
🌐 Modern Web UI — view HTML, source, headers, attachments, search & filter
📬 SMTP & POP3 servers — STARTTLS/SSL/TLS, authentication, forwarding, relaying
🔍 Testing tools — HTML check, link check, spam score, screenshots, mobile preview
🏷 Tagging & filtering — manual or automated (incl. plus addressing)
⚡ Performance — 100–200 emails/sec, automatic pruning (default 500 emails)
🧪 Chaos mode — inject SMTP errors to test resilience
🔔 Integrations — webhooks, browser notifications, List-Unsubscribe validation
🆕 Postmark API emulation — drop-in replacement for Postmark during development
🤖 MCP server — AI assistants (e.g., Claude Code) can list, search, and analyze messages

Installation

The web UI runs on http://0.0.0.0:8025 and SMTP on 0.0.0.0:1025.

Script (Linux & macOS)

sudo sh < <(curl -sL https://raw.githubusercontent.com/btafoya/mailsandbox/main/install.sh)

Static binary

Download from releases, rename to mailsandbox, and add to $PATH.

Docker

docker run -d --name mailsandbox   -p 8025:8025 -p 1025:1025   btafoya/mailsandbox

Usage

List options:

mailsandbox -h

Postmark API Emulation

Enable Postmark API emulation:

mailsandbox --postmark-api --postmark-token "your-secret-token"

Environment variables:

export MP_POSTMARK_API=true
export MP_POSTMARK_TOKEN=your-secret-token
mailsandbox

Example: Node.js

const postmark = require("postmark");
const client = new postmark.ServerClient("your-secret-token");
client.apiUrl = "http://localhost:8025";
client.sendEmail({ From:"[email protected]", To:"[email protected]", Subject:"Test", TextBody:"Hello" });

Endpoints:

POST /email
POST /email/batch
POST /email/withTemplate

MCP Server for AI Assistants

Enable MCP server:

mailsandbox --mcp-server --mcp-transport stdio

Environment variables:

export MP_MCP_SERVER=true
export MP_MCP_TRANSPORT=stdio

Docker with MCP

docker build -t mailsandbox-with-mcp .
docker run -d --name mailsandbox   -p 127.0.0.1:8025:8025 -p 1025:1025   -e MP_MCP_SERVER=true   -e MP_MCP_TRANSPORT=stdio   -e MP_POSTMARK_API=true   -e MP_POSTMARK_TOKEN=dev-token-123   mailsandbox-with-mcp

MCP tools

list_messages — list/filter emails
get_message — retrieve full message
search_messages — advanced search
analyze_message — HTML, links, spam scoring

Migration Guide

Binary name renamed mailsandbox; create a symlink if you prefer mailpit.
Default ports unchanged: HTTP 8025, SMTP 1025.
APIs: REST API unchanged; Postmark & MCP are opt-in.
Docker: use forked image for MCP/Postmark features.

Credits & Acknowledgements

Mailpit — by Axel Lenferna de la Motte and contributors. Original project and active development.
MailHog — the original inspiration for mainer email testing tools.
MailSandbox — maintained by Brian Tafoya, adding Postmark API emulation and MCP server support.

License

MailSandbox inherits the upstream license from Mailpit. See LICENSE for details.

mailsandbox