Lettio MCP

Let your AI assistant read, search and send email from a private, EU-hosted mailbox.

Lettio MCP is a Model Context Protocol server that connects an AI agent (Claude, Cursor, and any other MCP client) to a Lettio mailbox over JMAP. Your assistant can triage your inbox, find a message, and send a reply.

• 🇪🇺 European by default — talks to your own EU-hosted mailbox, no third‑party middleman.

• 🔒 Safe sending — the server can only ever send as the mailbox it is signed in to. It cannot be tricked into sending from another address.

• 📬 Everything an agent needs — list, search, read and send, over the open JMAP standard.

• 👥 One or many mailboxes — configure a single account or several.

Prefer nothing to install? Lettio also runs as a hosted, OAuth‑secured MCP server at https://mcp.lettio.eu/mcp — add that URL in your client and sign in. This npm package is the local / self‑hosted option for people who want to run it on their own machine.

Hosted server: mail cards in Claude

The hosted server at https://mcp.lettio.eu/mcp adds three tools that open interactive cards in clients that support MCP Apps, such as Claude:

• show_emails: the inbox, a folder or search results as a list you can open, archive and flag

• show_email: a whole conversation, with Reply, Reply all, Archive and Trash

• preview_email: a drafted message you can edit, preview in your brand and send

Sending from a card always needs your click, even when the connection doesn't allow the agent to send on its own. One connection can cover up to 10 mailboxes; searches run across all of them.

Related MCP server: Fastmail MCP Server (Unofficial)

Tools

Messages are written in Markdown and rendered as a styled email: headings, tables, quotes (end with > — Name), callouts (> [!NOTE] Title), code blocks, numbered steps and buttons ([Label](https://url "button")). The mailbox signature is added automatically; set it with set_signature.

To match your brand, set LETTIO_BRANDS to JSON (or a path to a JSON file) keyed by domain:

{ "example.com": { "primary": "#231F47", "accent": "#D8093A", "font": "Lexend, Arial, sans-serif" } }

Formatted mail goes out as HTML with a plain-text version. Pass format: "plain" to send text as written. Bulk tools take up to 100 ids.

Attachments can be base64 content, a local path, or from_email_id with attachment_id to reuse a file from another message.

Requirements

• Node.js 18 or newer.

• A Lettio mailbox and an app password for it (use a dedicated app password, not your main login).

Use with Claude Desktop

Add this to your claude_desktop_config.json:

{
  "mcpServers": {
    "lettio": {
      "command": "npx",
      "args": ["-y", "@lettio/mcp"],
      "env": {
        "LETTIO_USERNAME": "you@yourcompany.eu",
        "LETTIO_PASSWORD": "your-app-password"
      }
    }
  }
}

Restart Claude Desktop; the Lettio tools appear in the tools menu.

Configuration

Configured entirely through environment variables.

Single mailbox

Several mailboxes

Set LETTIO_ACCOUNTS to a JSON array and omit the single‑account variables:

[
  { "name": "work", "username": "you@yourcompany.eu", "password": "app-password" },
  { "name": "sales", "username": "sales@yourcompany.eu", "password": "app-password" }
]

Then pass account: "work" to any tool to choose which mailbox to use.

Security

Sending is deliberately constrained. Before it sends, the server asks the mail host for the identities the signed‑in mailbox is allowed to use and requires one that matches the login address. The From header and the SMTP envelope MAIL FROM are both pinned to that address. There is no parameter for choosing a different sender, so an agent can never send "from" a foreign or arbitrary mailbox. Credentials are read from the environment, kept only in memory, and never logged.

Run from source

npm install
npm run build
LETTIO_USERNAME=you@yourcompany.eu LETTIO_PASSWORD=app-password npm start

License

MIT © Valmia Solutions s.r.o.