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.

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.

Related MCP server: gmail-mcp

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)

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

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.