Smtp Wrapper

A minimal, self-hosted MCP server that exposes a single send_email tool. It sends real HTML email through an SMTP relay (e.g. Gmail). The tool is served over the streamable-HTTP MCP transport at /mcp, with an unauthenticated /healthz liveness route. The implementation is intentionally tiny (stdlib smtplib) to keep the audit/attack surface small.

⚠️ Security requirement: this server MUST be gated by an authorization service

This server implements no authentication of its own, by design. Anyone who can reach /mcp can send email. Do not expose it directly to the internet or bind it to a public port.

It must sit behind an identity-aware authorization proxy — such as Pomerium in MCP mode, or an equivalent like Cloudflare Access or oauth2-proxy — that authenticates and authorizes every request before it reaches /mcp.

Reference topology:

edge tunnel → reverse proxy (TLS) → Pomerium (SSO + allowlist to a single identity) → smtp-mcp-wrapper
                                                                                        (internal network only)

The provided docker-compose.yml deliberately publishes no host ports and attaches the container only to the proxy's internal Docker network, so the server is unreachable except through the authorization proxy.

Defense in depth already built in (these complement, they do not replace, the proxy):

• ALLOWED_TO hard-limits recipients, so even a misused tool cannot mail outside the allowlist.
• Setting REQUIRE_POMERIUM_IDENTITY=true makes the app cryptographically verify Pomerium's identity assertion on every /mcp request — signature (against Pomerium's JWKS), expiry, and audience. This blocks anything on the shared Docker network from reaching the app directly and bypassing Pomerium. See Enabling app-layer verification.

Configuration

All configuration is via environment variables. Copy .env.example to .env and fill in real values. .env is git-ignored and must stay that way — it holds the SMTP password. Nothing secret is baked into the image (credentials are injected at runtime), which is why the published container image can safely be public.

Variable	Default	Description
SMTP_HOST	smtp.gmail.com	SMTP relay host.
SMTP_PORT	587	SMTP relay port (STARTTLS).
SMTP_USER	—	SMTP username.
SMTP_PASS	—	SMTP password. For Gmail, use an App Password.
MAIL_FROM	SMTP_USER	From address.
MAIL_FROM_NAME	—	Optional display name for the From header.
DEFAULT_TO	—	Recipient used when the tool's to argument is omitted.
ALLOWED_TO	—	Comma-separated recipient allowlist. Empty = any recipient allowed.
STARTUP_TEST_EMAIL	false	Send a test email to DEFAULT_TO on startup to verify SMTP. On failure, logs the SMTP error reason (auth/connection); the server keeps running either way.
REQUIRE_POMERIUM_IDENTITY	false	Verify Pomerium's identity assertion on every /mcp request (see below). Requires POMERIUM_JWKS_URL.
POMERIUM_JWKS_URL	—	Pomerium's JWKS endpoint, e.g. https://<host>/.well-known/pomerium/jwks.json. Required when the gate is on.
POMERIUM_AUDIENCE	—	Expected aud claim (the route host/URL). Verified when set — strongly recommended.
POMERIUM_ISSUER	—	Expected iss claim. Verified only when set.
POMERIUM_IDENTITY_HEADER	x-pomerium-assertion,x-pomerium-jwt-assertion	Comma-separated header(s) carrying the assertion JWT.
HOST / PORT	0.0.0.0 / 8080	Server bind address/port.

The send_email tool

send_email(subject: str, html: str, to?: str, text?: str) -> str

Sends an HTML email. to falls back to DEFAULT_TO and must be within ALLOWED_TO when that allowlist is set. text is an optional plain-text alternative for non-HTML clients.

Enabling app-layer verification

This step is optional — Pomerium already gates all access. Enable it only if you also want the app to reject any request that reaches it without a valid Pomerium identity (e.g. a compromised neighbor on the shared Docker network hitting email-mcp:8080 directly). When on, the app verifies the assertion JWT's signature, expiry, and audience.

1. Pomerium — set these on the email-mcp route. The critical addition is pass_identity_headers: true; without it Pomerium forwards no identity header and the app rejects every request. Pomerium must also have a signing key configured (it serves the matching public keys at /.well-known/pomerium/jwks.json).

routes:
  - from: https://email-mcp.example.com
    to: http://email-mcp:8080        # pathless — the /mcp path passes through
    name: email-mcp
    mcp:
      server: {}
    pass_identity_headers: true       # <-- REQUIRED: sends X-Pomerium-Assertion to the app
    policy:
      - allow:
          and:
            - email:
                is: [email protected]

2. App — set these in .env:

REQUIRE_POMERIUM_IDENTITY=true
POMERIUM_JWKS_URL=https://email-mcp.example.com/.well-known/pomerium/jwks.json
POMERIUM_AUDIENCE=email-mcp.example.com

Then docker compose up -d. If REQUIRE_POMERIUM_IDENTITY=true but POMERIUM_JWKS_URL is unset, the server refuses to start (a security gate must not run unable to verify). To turn the feature off again, set REQUIRE_POMERIUM_IDENTITY=false.

Run

cp .env.example .env      # then edit .env with real values
docker compose up -d

Health check:

docker compose exec email-mcp \
  python -c "import urllib.request; print(urllib.request.urlopen('http://localhost:8080/healthz').read())"
# -> b'ok'

Then add the email-mcp route to your authorization proxy (pathless upstream, e.g. to: http://email-mcp:8080, so the /mcp path passes through) and connect your MCP client to https://<your-host>/mcp.

Maintenance

Patches flow with near-zero manual effort:

• Dependabot (.github/dependabot.yml) watches requirements.txt, the Dockerfile base image, and the workflow's actions, opening upgrade PRs weekly. Also enable Dependabot security updates in the repo's Settings → Code security.
• CI (.github/workflows/build.yml) builds and pushes the image to GHCR on push to main, on Dependabot PRs, via manual dispatch, and weekly (Mon 06:00 UTC) with no-cache so the OS and Python patches are genuinely refreshed even without code changes.
• On the host, pull the rebuilt image with Watchtower (the compose file already sets the opt-in label) or a cron running docker compose pull && docker compose up -d.

Links

• Pomerium — MCP support
• Pomerium — Protect an MCP server
• Dependabot configuration options