JMAP mail

jmap-mcp

An MCP server that gives an AI assistant (Claude.ai, Claude Code, Claude Desktop or any MCP client) access to a mailbox hosted on a JMAP server (RFC 8620, RFC 8621), first of all Stalwart.

The user gives an email address and signs in on their mail server's own page. Nothing else is configured: no server URL, no token, no API key.

Why

• Sovereignty. Your mail stays on your server. The bridge is self-hosted, stores no email, and keeps only encrypted OAuth tokens.
• Zero configuration. The JMAP server is found from the email address (DNS SRV, then /.well-known/jmap), its OAuth server from standard metadata, and the bridge registers itself dynamically. Secrets are generated on first start.
• Clean OAuth, no token passthrough. MCP clients get tokens issued by the bridge for the bridge; mail-server tokens never leave it, as the MCP authorization spec requires. It works as a Claude.ai custom connector with the Client ID left empty.
• Any JMAP server. Nothing is Stalwart-specific: any server with JMAP and an OAuth server supporting RFC 8414 metadata and RFC 7591 dynamic registration works.

Existing JMAP MCP servers (mikluko/jmap-mcp, wyattjoh/jmap-mcp, agent-jmap-mcp, fastmail-blade-mcp) all need a server URL and a static token in their configuration, and none can be added to Claude.ai as a connector.

Tools

A small surface designed for an agent: compact JSON results, pagination everywhere, bodies converted to text and truncated with an explicit flag.

The resource jmap-mcp://status describes the connected accounts, the discovered servers and the capabilities they announce.

Email content is treated as untrusted data: it is returned to the model as data, never interpreted, and the tool descriptions tell the model that emails may contain instructions it must ignore.

Choices that are not secret (which account, which sender identity, confirmation of a send or a permanent deletion) are asked through MCP elicitation when the client supports it. Otherwise the tool returns an error listing the choices, and the model calls it again with the argument set (account, identity, confirm). Credentials never go through elicitation or tool arguments.

How it works

                         HTTP mode                                     mail server

┌──────────────┐ OAuth 2.1 (bridge tokens) ┌──────────────┐ OAuth (mail tokens) ┌──────────────┐ │ MCP client │ ──────────────────────────▶ │ jmap-mcp │ ────────────────────▶ │ OAuth server │ │ Claude.ai… │ ◀── MCP over HTTP ────────▶ │ (bridge) │ ◀──── JMAP ─────────▶ │ JMAP API │ └──────────────┘ └──────────────┘ └──────────────┘

In HTTP mode (jmap-mcp serve) the bridge is at the same time:

1. a protected resource (RFC 9728 metadata, 401 with WWW-Authenticate, audience checked on every request);
2. a minimal authorization server for MCP clients (RFC 8414 metadata, PKCE S256, RFC 8707 resource indicators, RFC 7591 dynamic registration, Client ID Metadata Documents, rotating refresh tokens);
3. an OAuth client of the mail server: its sign-in page asks for the email address, discovers the server, registers itself there once per server, and redirects the user to the mail server's authorization code flow with PKCE. Only after the mail server returns does the bridge issue its own code and tokens.

A single deployment serves users of different mail servers. Each user gets an isolated MCP server instance and can only reach their own accounts.

In local mode (jmap-mcp, stdio) there is no third party: the process is the OAuth client of the mail server, like a mail application. It registers as a public client, opens the browser on a loopback redirect, and falls back to the device flow (RFC 8628) when no browser can be opened, showing the code to type.

Installation

1. Docker behind Traefik

The image is ghcr.io/kolapsis/jmap-mcp (linux/amd64 and linux/arm64, distroless, non-root, data in the /data volume). With a Traefik that already has a websecure entrypoint and an ACME resolver, copy docker-compose.yml, set your host name, resolver and network, and start it:

curl -O https://raw.githubusercontent.com/kOlapsis/jmap-mcp/main/docker-compose.yml

edit mcp.example.com, the certresolver name and the network

docker compose up -d curl https://mcp.example.com/healthz

Traefik sets the X-Forwarded-* headers from which the bridge derives its public URL. Set JMAP_MCP_PUBLIC_URL only if your proxy does not send them.

Image tags: X.Y.Z, X.Y, X and latest for releases, main and sha- for the latest build of the main branch. Both are built by GitHub Actions and pushed to ghcr.io.

Without Traefik, deploy/caddy has a Caddy setup that obtains the certificate itself:

cd deploy/caddy   # edit mcp.example.com in the Caddyfile

docker compose up -d

The MCP address is https://mcp.example.com/mcp.

2. Add it to Claude.ai

Settings › Connectors › Add custom connector. Give it a name and the URL https://mcp.example.com/mcp. Leave Client ID and Client Secret empty in the advanced settings: Claude.ai registers itself dynamically (its redirect URI is https://claude.ai/api/mcp/auth_callback). Click Connect, enter your email address on the bridge page, then sign in on your mail server.

The free plan is limited to one custom connector. On Team and Enterprise plans, an owner adds the connector under Organization settings › Connectors.

3. Claude Code with the remote bridge

claude mcp add --transport http mail https://mcp.example.com/mcp

Then run /mcp in Claude Code and choose the server to authenticate in the browser.

4. Claude Code and Claude Desktop, local (stdio)

Install the binary, from the releases, with Homebrew (brew install --cask kolapsis/tap/jmap-mcp, macOS and Linux) or with Go:

go install github.com/kOlapsis/jmap-mcp@latest

Claude Code:

claude mcp add mail -- jmap-mcp

or in .mcp.json:

{

"mcpServers": { "mail": { "command": "jmap-mcp", "args": ["stdio"] } } }

Claude Desktop: install jmap-mcp.mcpb from the release (double-click it, or Settings › Extensions), or add to claude_desktop_config.json the full path of the binary:

{

"mcpServers": { "mail": { "command": "/usr/local/bin/jmap-mcp", "args": ["stdio"] } } }

On first use, ask the assistant to connect your mailbox. It asks for your email address (elicitation, or the connect_account tool), opens your browser on the mail server's sign-in page, and the account is ready. Without a graphical session (SSH), it shows a URL and a code to enter on any device. Tokens are stored in the data directory (~/.config/jmap-mcp on Linux), encrypted with a key kept in the OS keyring, or in a 0600 key file when no keyring is available. Several accounts can be connected.

To send email in local mode, add "env": {"JMAP_MCP_ENABLE_SEND": "true"} to the server entry.

5. Binary with systemd

sudo install -m 755 jmap-mcp /usr/local/bin/

sudo cp deploy/systemd/jmap-mcp.service /etc/systemd/system/ sudo systemctl daemon-reload && sudo systemctl enable --now jmap-mcp

The unit listens on 127.0.0.1:8080 with a dynamic user and its state in /var/lib/jmap-mcp. Put a TLS reverse proxy in front of it (any proxy that sends X-Forwarded-Proto and X-Forwarded-Host), or set JMAP_MCP_PUBLIC_URL.

Preparing the mail server

Discovery. The JMAP server is found from the email domain, as RFC 8620 §2.2 describes: an SRV record _jmap._tcp.example.com pointing at the JMAP host, otherwise https://example.com/.well-known/jmap. If your web site is not served by the mail server, add the SRV record:

_jmap._tcp.example.com. 3600 IN SRV 0 1 443 mail.example.com.

The OAuth server is found from the JMAP host: RFC 9728 resource metadata, then RFC 8414 and OpenID Connect discovery. It must be under the same registrable domain as the JMAP host (for example jmap.example.com and auth.example.com); otherwise the bridge refuses the server.

Client registration with Stalwart. Nothing to do with the default settings: the bridge registers itself as a public client with PKCE, which Stalwart accepts anonymously (anonymousClientRegistration, on by default in Stalwart 0.16). Stalwart only issues confidential clients to authenticated callers, so the bridge does not get a client secret this way; PKCE protects the authorization code instead.

If the operator turned anonymous registration off, declare the client statically:

1. In Stalwart, Management › Directory › OAuth Clients, create a client with the redirect URI https://mcp.example.com/callback (and a secret if you want a confidential client).
2. Get the admin token of the bridge: docker compose exec jmap-mcp /jmap-mcp admin-token (or `jmap-mcp admin-token --d