Mailpit Server

A Model Context Protocol server for Mailpit, the email & SMTP testing tool. It lets AI assistants (Claude Code, Claude Desktop, Cursor, VS Code, …) read, search and manage the emails your application sends during development and testing.

• Streamable HTTP transport — run it as a network service next to Mailpit (Docker Compose friendly), or
• stdio transport — let your MCP client spawn it locally
• Bearer-token auth for the HTTP endpoint, so it can safely sit behind a reverse proxy
• Single small dependency surface: the official @modelcontextprotocol/sdk, Express and Zod

Tools

Tool	Description
list_messages	List mailbox messages, newest first (paginated)
search_messages	Search with Mailpit query syntax (from:, subject:, is:unread, tag: …)
get_message	Full message by ID or latest — headers, text/HTML bodies, attachment metadata
get_message_headers	Raw headers of a message
get_message_source	Full raw RFC-822 source (headers + MIME parts)
get_attachment	Download an attachment — images returned as images, text as text, other as base64
check_html	HTML compatibility analysis across email clients
get_message_links	Extract all URLs without requesting them — safe retrieval of confirmation/reset/unsubscribe links for e2e flows
check_links	Verify links by requesting them (real GETs — may trigger one-click actions; auth-protected links can return 302/401/403 without being broken)
send_message	Compose and inject a test email via the Mailpit API (captured, not delivered)
send_smtp_message	Send over a REAL SMTP transaction (as applications do) to prove the SMTP channel works end-to-end
wait_for_message	Poll until a new (optionally matching) message arrives — great for e2e flows
set_read_status	Mark messages read/unread, by ID or all
delete_messages	Delete messages by ID (set all:true to delete every message)
get_mailbox_info	Mailpit version, message/unread counts, database size

Quick start (Docker Compose)

services:
  mailpit:
    image: axllent/mailpit:latest
    ports:
      - "8025:8025"   # web UI
      - "1025:1025"   # SMTP
    healthcheck:
      test: ["CMD", "/mailpit", "readyz"]
      interval: 30s
      start_period: 10s

  mailpit-mcp:
    image: ghcr.io/cedjey/mailpit-mcp-server:2
    ports:
      - "3000:3000"   # MCP endpoint: http://localhost:3000/mcp
    environment:
      MAILPIT_URL: http://mailpit:8025
      # Required: where applications send SMTP, as reachable by them
      MAILPIT_SMTP_ENDPOINT: localhost:1025
      # Recommended: require "Authorization: Bearer <token>" on /mcp
      # MCP_AUTH_TOKEN: your-secret-token
    depends_on:
      mailpit:
        condition: service_healthy

Or standalone against an existing Mailpit:

docker run -d -p 3000:3000 \
  -e MAILPIT_URL=http://your-mailpit-host:8025 \
  -e MAILPIT_SMTP_ENDPOINT=your-mailpit-host:1025 \
  -e MCP_AUTH_TOKEN=your-secret-token \
  ghcr.io/cedjey/mailpit-mcp-server:2

Connecting clients

Claude Code:

claude mcp add --transport http mailpit http://localhost:3000/mcp \
  --header "Authorization: Bearer your-secret-token"

.mcp.json / Cursor / VS Code:

{
  "mcpServers": {
    "mailpit": {
      "type": "http",
      "url": "http://localhost:3000/mcp",
      "headers": { "Authorization": "Bearer ${MAILPIT_MCP_TOKEN}" }
    }
  }
}

Claude Desktop (stdio):

{
  "mcpServers": {
    "mailpit": {
      "command": "docker",
      "args": ["run", "-i", "--rm",
        "-e", "MCP_TRANSPORT=stdio",
        "-e", "MAILPIT_URL=http://host.docker.internal:8025",
        "-e", "MAILPIT_SMTP_ENDPOINT=localhost:1025",
        "ghcr.io/cedjey/mailpit-mcp-server:2"]
    }
  }
}

Or without Docker (Node 20+): clone the repo, npm ci, and use "command": "node", "args": ["/path/to/server.js"] with MCP_TRANSPORT=stdio.

Built-in agent context

The server ships the context an AI agent needs — no extra prompting required by consumers:

• Server instructions (delivered via the MCP initialize handshake): what the mailbox is, the standard workflows (inspect, e2e wait-then-verify, template QA, attachments), search syntax, and warnings — clients inject these into the model's context automatically.
• Tool annotations: the read-only tools carry readOnlyHint; the side-effecting ones (send_message, send_smtp_message, set_read_status, check_links, delete_messages) do not — so clients can auto-approve reads and gate writes. check_links is deliberately not read-only (it performs real GETs and can trigger one-click links); delete_messages additionally carries destructiveHint.
• MCP prompts: verify_email (wait for a message matching a query, then verify structure, links and client compatibility against an optional checklist) and inspect_mailbox (summarize mailbox state, flag anomalies). Claude Code exposes these as slash commands.

Claude Code plugin

The easiest way to use this from Claude Code — one install, prompted for your endpoint and token, no manual MCP or skill setup:

/plugin marketplace add CeDJeY/mailpit-mcp-server
/plugin install mailpit@cedjey

On enable, Claude Code asks for:

• Mailpit MCP endpoint URL — e.g. http://your-host:3000/mcp
• Bearer token — the server's MCP_AUTH_TOKEN (leave empty if auth is disabled)

The plugin bundles the MCP connection plus two skills:

• verify-email — the verification workflow: wait for the email, inspect structure, extract links safely, check HTML compatibility, report a PASS/FAIL verdict.
• route-mail-to-mailpit — the prerequisite the verification depends on: configure the application under test to send SMTP to Mailpit, prove it with a canary email, and flag configurations that could leak test mail to real SMTP providers.

Plugin source lives in plugin/; non-plugin users can copy the skills into their project's .claude/skills/ manually.

Configuration

Variable	Default	Description
MAILPIT_URL	http://localhost:8025	Base URL of the Mailpit instance
MCP_TRANSPORT	http	http (network service) or stdio (spawned by client)
MCP_HTTP_PORT	3000	Port for the HTTP endpoint
MCP_AUTH_TOKEN	(empty = auth off)	When set, /mcp requires Authorization: Bearer <token>
MAILPIT_AUTH_USER / MAILPIT_AUTH_PASS	(none)	Basic-auth credentials if Mailpit's API is protected
MAILPIT_SMTP_ENDPOINT	required	SMTP host:port applications must send to (as reachable by them); exposed to agents via get_mailbox_info (SMTPEndpoint) and the server instructions for routing checks, and used as send_smtp_message's default target. The server refuses to start without it (MAILPIT_SMTP_ADVERTISE still accepted as a deprecated alias)
MAILPIT_SMTP_ALLOWED_ENDPOINTS	(empty = any host)	Comma-separated allowlist of host:port targets accepted by send_smtp_message's endpoint override. MAILPIT_SMTP_ENDPOINT is always allowed; the link-local/metadata range (169.254.0.0/16) is always blocked

The HTTP mode also serves GET /healthz (no auth) for container healthchecks.

Security notes

Mailpit captures every email your app sends — treat the MCP endpoint as sensitive. Set MCP_AUTH_TOKEN whenever the port is reachable beyond localhost, and prefer keeping it off the public internet (private network or VPN). delete_messages is destructive; scope access accordingly.

Development

npm ci
MAILPIT_URL=http://localhost:8025 npm start   # HTTP mode on :3000
node mcp-test.js                              # e2e check against a running server

mcp-test.js honours MCP_URL and MCP_AUTH_TOKEN.

License

MIT