Outbox

FieldCure MCP Outbox Server

A multi-channel messaging Model Context Protocol (MCP) server that sends messages through Slack, Telegram, Email (Gmail, Naver, Microsoft Graph API), KakaoTalk, and Discord. Built with C# and the official MCP C# SDK.

Features

• Multiple messaging channels — Slack, Telegram, Email (Gmail, Naver, Microsoft Graph API), KakaoTalk, Discord
• 4 MCP tools — list_channels, add_channel, send_message, remove_channel
• Cross-platform credential flow — runtime secrets can come from env vars or MCP elicitation; OAuth tokens are stored separately for refresh lifecycle
• CLI channel setup — interactive console for browser/OAuth flows where applicable
• SMTP presets — Gmail, Naver with one-command setup
• Microsoft Graph API — OAuth 2.0 browser flow for Outlook / M365 email with automatic token refresh
• KakaoTalk OAuth — localhost callback flow with automatic token refresh
• Telegram Client API — send to Saved Messages via WTelegramClient
• Stdio transport — standard MCP subprocess model via JSON-RPC over stdin/stdout

Why Outbox?

Existing MCP servers are channel-specific — one for Slack, another for Gmail, yet another for Telegram. Each requires separate installation, configuration, and the LLM must know which tool to call for each channel.

Outbox takes a different approach:

• One tool, multiple channels — send_message abstracts away channel differences. The LLM doesn't need to know Slack API vs SMTP vs Kakao REST.
• Credential isolation — OAuth refresh tokens live in tokens.json with current-user-only file permissions; MCP tool flows can resolve secrets at runtime via env vars or elicitation.
• Single install — dotnet tool install -g gives you 4 channels. No need to install and configure separate servers per channel.
• KakaoTalk support — Currently the only MCP server with KakaoTalk messaging, essential for Korean users.

Installation

dotnet tool (recommended)

dotnet tool install -g FieldCure.Mcp.Outbox

After installation, the fieldcure-mcp-outbox command is available globally.

From source

git clone https://github.com/fieldcure/fieldcure-mcp-outbox.git
cd fieldcure-mcp-outbox
dotnet build

Requirements

• .NET 8.0 Runtime or later
• Cross-platform (Windows, Linux, macOS)

Configuration

Claude Desktop

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "outbox": {
      "command": "fieldcure-mcp-outbox"
    }
  }
}

VS Code (Copilot)

Add to .vscode/mcp.json:

{
  "servers": {
    "outbox": {
      "command": "fieldcure-mcp-outbox"
    }
  }
}

From source (without dotnet tool)

{
  "mcpServers": {
    "outbox": {
      "command": "dotnet",
      "args": [
        "run",
        "--project", "C:\\path\\to\\fieldcure-mcp-outbox\\src\\FieldCure.Mcp.Outbox"
      ]
    }
  }
}

Tools

Channels

OAuth tokens for Microsoft/KakaoTalk are stored in tokens.json so the server can refresh them across runs (current-user-only file permissions — Windows ACL / Unix 0600).

Static secrets (Slack bot tokens, Discord webhook URLs, SMTP passwords, etc.) resolve at send time in this order: in-memory cache → env var OUTBOX_{CHANNEL_ID}_{FIELD} → plaintext in channels.json → MCP elicitation. The CLI add_channel flow writes directly into channels.json for a zero-configuration local experience. This is an intentional local-trust choice with the same same-user boundary as tokens.json, documented in ADR-001 Principle 2. For shared hosts, CI, or headless deployments, set the env vars explicitly and leave channels.json secret fields empty.

Static Secret Environment Variables

When a channel requires a static secret at send time, Outbox looks for an environment variable named:

OUTBOX_<CHANNEL_ID>_<FIELD>

Examples:

• OUTBOX_MICROSOFT_1_CLIENT_SECRET
• OUTBOX_KAKAOTALK_1_API_KEY
• OUTBOX_KAKAOTALK_1_CLIENT_SECRET
• OUTBOX_SMTP_GMAIL_1_PASSWORD

If the variable is unset and the MCP client supports elicitation, Outbox prompts for it interactively and caches it for the current session.

CLI Commands

fieldcure-mcp-outbox                      # Start MCP server (stdio)
fieldcure-mcp-outbox add slack            # Add Slack channel
fieldcure-mcp-outbox add telegram         # Add Telegram channel
fieldcure-mcp-outbox add gmail            # Add Gmail SMTP channel
fieldcure-mcp-outbox add naver            # Add Naver SMTP channel
fieldcure-mcp-outbox add smtp             # Add custom SMTP channel
fieldcure-mcp-outbox add microsoft        # Add Microsoft (Outlook/M365) channel
fieldcure-mcp-outbox add kakaotalk        # Add KakaoTalk channel
fieldcure-mcp-outbox add discord          # Add Discord channel
fieldcure-mcp-outbox list                 # List configured channels
fieldcure-mcp-outbox remove <id>          # Remove a channel

Data Storage

tokens.json is stored in plain JSON and protected by filesystem permissions:

• Windows: ACL restricted to the current user
• Linux/macOS: mode 0600

This protects against other local users on the same machine, but not against compromise of the same OS user account.

Project Structure

src/FieldCure.Mcp.Outbox/
├── Program.cs                  # Entry point: MCP server vs CLI branching
├── Channels/
│   ├── IChannel.cs             # Channel interface + SendRequest/SendResult
│   ├── ChannelFactory.cs       # Channel instantiation by type
│   ├── SlackChannel.cs         # Slack Web API
│   ├── TelegramChannel.cs      # WTelegramClient
│   ├── SmtpChannel.cs          # MailKit SMTP
│   ├── MicrosoftChannel.cs     # Microsoft Graph API
│   ├── KakaoTalkChannel.cs     # Kakao REST API
│   └── DiscordChannel.cs       # Discord Webhook API
├── Tools/
│   ├── ListChannelsTool.cs     # list_channels
│   ├── AddChannelTool.cs       # add_channel (elicitation + browser OAuth for Kakao/Microsoft)
│   ├── RemoveChannelTool.cs    # remove_channel
│   └── SendMessageTool.cs      # send_message
├── Interaction/
│   ├── IElicitGate.cs          # Minimal MCP elicitation surface for tests
│   └── McpServerElicitGate.cs  # Production adapter around McpServer
├── OAuth/
│   └── BrowserOAuthFlow.cs     # Localhost callback + Process.Start + elicit race (MCP and CLI entry points)
├── Credentials/
│   └── OutboxSecretResolver.cs # cache → env → channels.json → elicitation
├── Setup/
│   ├── SetupRunner.cs          # CLI router (diagnostic path)
│   ├── ConsoleHelper.cs        # Masked input, prompts
│   ├── SlackSetup.cs
│   ├── TelegramSetup.cs
│   ├── SmtpSetup.cs
│   ├── MicrosoftSetup.cs
│   ├── KakaoTalkSetup.cs
│   └── DiscordSetup.cs
└── Configuration/
    ├── ChannelStore.cs         # channels.json persistence (metadata + static-secret fallback)
    ├── OAuthTokenStore.cs      # tokens.json persistence (OAuth access/refresh, user-only file perms)
    └── SmtpPresets.cs          # SMTP preset definitions

Development

# Build
dotnet build

# Test
dotnet test

# Pack as dotnet tool
dotnet pack src/FieldCure.Mcp.Outbox -c Release

See Also

Part of the AssistStudio ecosystem.

License

MIT