Mailbuttons (mbag.ai) MCP server

Mailbuttons MCP server

Governed email for AI agents, over the Model Context Protocol. Give an agent a scoped mailbox where the server enforces the guardrails, not the prompt: sandbox-by-default inboxes, a policy gate on every send, and a tamper-evident audit log. External sending and scope changes are human-approved, never granted by the agent itself.

• Registry: com.mailbuttons/mcp-server
• npm: @mailbuttons/mcp-server
• Platform: mailbuttons.com · agent front door: mbag.ai

Use it

Hosted (recommended) — a streamable-HTTP endpoint served by the platform:

https://mailbuttons.com/api/v1/mcp/rpc
Authorization: Bearer <your Mailbuttons MCP token>   # mb_sandbox_... (or mb_prod_...)

Local (stdio) — run the npm package and let your agent launch it:

{
  "mcpServers": {
    "mailbuttons": {
      "command": "npx",
      "args": ["-y", "@mailbuttons/mcp-server"],
      "env": { "MAILBUTTONS_API_KEY": "mb_sandbox_..." }
    }
  }
}

Get a scoped token from the dashboard or POST /api/v1/mcp/sandbox-inboxes. Sandbox tokens can never send externally until a human promotes them.

What the server enforces

bounces by default, so nobody can prompt-inject your agent just by emailing it.

• Inbound — a per-mailbox sender policy, fail-closed: mail from strangers

blocked send returns a normal {"status":"blocked"} result, not an error.

• Outbound — a recipient blocklist the agent can read but not change; a

records refusals too.

• Caps + audit — per-account send caps and a hash-chained audit log that

named human approves. The agent cannot approve its own request.

• Escalation — sending externally or widening scope is propose-only; a

Install as an agent skill

This repo ships a root SKILL.md, so any skills-aware agent can add it:

npx skills add mailbuttons/mcp-server

Docs

• Agent signup (for agents): https://mbag.ai/docs/agent-signup.md
• Trust model: https://mbag.ai/docs/trust-model.md
• OpenAPI: https://mbag.ai/api/openapi.json
• Framework recipes: references/ (Claude Agent SDK, LangChain, plain SDK)

Mailbuttons is a trading name of Code Cutter Limited (UK, no. 08453060). MIT licensed.