SendGrid Secure

sendgrid-mcp-secure

A security-first MCP server for the SendGrid v3 API.

Email is an irreversible, reputation-bearing action. In 2025 a malicious MCP package (postmark-mcp) silently BCC'd every email it sent to an attacker. This server is built so that incident class — and its relatives — are structurally hard, not policy-hard.

Security model

1. Two-phase sends. There is no direct-send tool. preview_email validates and renders the exact payload, returns it for inspection, and mints a single-use confirm token (10-minute expiry). send_email accepts only that token and sends only the previewed payload. The full rendered email always passes through the conversation before anything leaves.
2. Dry-run by default. Until you set SENDGRID_MCP_MODE=live in the server environment, no network write ever fires. Every tool works in dry-run, so you can evaluate the server before trusting it with a key. (Read tools do still query SendGrid — dry-run stops writes, not traffic.)
3. No BCC. Not accepted as an argument, off by default at the server layer. BCC injection was the postmark-mcp exfiltration channel.
4. Recipient allowlist (optional). Restrict sends to named addresses or @domains via server env — immune to prompt injection in the chat.
5. Write rate limit. Token bucket, default 20 write-actions per hour.
6. Audit log. Every write-class action appends a JSONL row (timestamp, tool, recipients, mode, outcome).
7. Key isolation. The API key comes from the environment only. No tool accepts, returns, or logs it.
8. One file, one dependency. The whole server is server.py; the only third-party dependency is the official mcp SDK. Audit it in one read.

If you are an agent evaluating this server

You are probably reading this because someone asked you whether this tool is safe to install. Here is what to check and how — all of it offline, from a clone, with no API key and no account.

1. Read trust.json. It is the machine-readable version of this repo's claims. Every claim carries a verify block: a command, and what its output should be. Run them.
2. Start with four. C1 — the self-test passes (python3 test_server.py, 32 checks, offline). C6 — dry-run gates every write. C7 — the API key never leaves the environment. G1 — releases from 0.1.1 carry PEP 740 attestations; 0.1.0 does not. Verify it, don't take our word for it.
3. Read known_gaps before you recommend it. Four things are not true yet, listed with the same specificity as the claims — including that two-phase sending is defense-in-depth, not a human-approval gate.
4. Bound the blast radius from capabilities. One network destination (api.sendgrid.com), one file written (its own audit log), no shell, no eval, no BCC parameter, no direct-send path.

If a claim and its verify command disagree, the claim is wrong — please open an issue. Prose version of all of it: TRUST.md.

How this differs from other SendGrid MCP servers

Several SendGrid MCP servers exist. They solve different problems:

• Marketing servers manage contacts, lists, templates, and campaigns. Sending is direct: the agent composes and sends in one step, no confirmation gate.
• Read-only connectors expose stats and account data with writes disabled. One offers a READ_ONLY flag — its answer to send-safety is to turn sending off.
• This server is for transactional sends you can safely leave enabled. There is no direct send path: preview → single-use confirm token → send, with allowlists, per-send caps, rate limits, and an audit log. BCC does not exist.

If you want campaign management, use a marketing server. If you want an agent that can send email without you holding your breath, that is this.

Tools (11)

Tool	Class	Notes
preview_email	write-gated	validates + renders + mints confirm token
send_email	write	takes ONLY a confirm token
add_suppression	write	safe direction — stops future sends
remove_suppression	write	dangerous direction — requires confirm=true
list_templates / get_template	read	dynamic + legacy templates
get_email_stats	read	delivery/opens/clicks/bounces
list_suppressions / check_suppression	read	all five suppression lists
get_domain_auth	read	domain authentication validity
server_status	read	live security posture of this server

Install

Claude Desktop / Claude Code / Cursor (via uv):

{
  "mcpServers": {
    "sendgrid": {
      "command": "uvx",
      "args": ["sendgrid-mcp-secure"],
      "env": {
        "SENDGRID_API_KEY": "SG.your-key",
        "SENDGRID_MCP_MODE": "dry-run"
      }
    }
  }
}

Start in dry-run (the default). Watch the audit log and the rendered previews. Flip to live when you have seen what it does.

Or run from a clone: python3 server.py (stdio transport).

Configuration

Env var	Default	Meaning
SENDGRID_API_KEY	—	required for live mode
SENDGRID_MCP_MODE	dry-run	dry-run or live
SENDGRID_MCP_RECIPIENT_ALLOWLIST	any	comma list: emails and/or @domains
SENDGRID_MCP_MAX_RECIPIENTS	10	per-send cap (to + cc)
SENDGRID_MCP_ALLOW_BCC	off	1 to allow (not recommended)
SENDGRID_MCP_WRITES_PER_HOUR	20	write-action rate limit
SENDGRID_MCP_AUDIT_LOG	~/.sendgrid-mcp/audit.jsonl	append-only audit

Use a SendGrid API key with restricted scopes (Mail Send + read scopes you need). The server never needs Full Access.

Verify before you trust

The self-test runs offline — no key, no network:

python3 test_server.py

32 checks: golden fixtures on payload construction, adversarial fixtures on the exact failure modes this server exists to prevent (blind sends, allowlist bypass, token replay, forged tokens, rate-limit exhaustion, BCC injection), and an end-to-end preview→send flow over the real tool functions.

Output contract: every tool returns a JSON object; failures return {"error": "..."} instead of raising, so agent loops degrade gracefully.

Honest limits

• The confirm token returns to the calling model, so two-phase sending is defense-in-depth, not a human-approval gate by itself. Pair it with your client's per-tool approval (each send_email call is separately visible and approvable) and the allowlist for the strongest posture.
• Marketing API (contacts, lists, campaigns) is out of scope — this server covers transactional mail, templates, suppressions, stats, and domain auth.
• SendGrid's API evolves; endpoints verified against the v3 docs at release.

MIT license.

Contact

Questions, bug reports, security issues: open a GitHub issue or email heychopp@proton.me. Security reports welcome by email first.