FieldCure Outbox Server

NuGet License: MIT

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

Tool	Description	Confirmation
list_channels	List all configured messaging channels	—
add_channel	Add a new channel (MCP elicitation for all channel types; Microsoft/KakaoTalk require a local browser on the MCP host)	—
send_message	Send a message through a configured channel	Required
remove_channel	Remove a channel and its stored credentials	Required

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.

Channel	Protocol	Setup
Slack	Web API (chat.postMessage)	Guide
Telegram	Client API (WTelegramClient)	Guide
Gmail	SMTP	Guide
Naver	SMTP	Guide
Microsoft	Graph API (/me/sendMail)	Guide
KakaoTalk	Kakao REST API	Guide
Discord	Webhook API	Guide
Custom SMTP	User-defined	Guide

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

Data	Location
Channel metadata	%LOCALAPPDATA%\FieldCure\Mcp.Outbox\channels.json
OAuth tokens	%LOCALAPPDATA%\FieldCure\Mcp.Outbox\tokens.json
Telegram sessions	%LOCALAPPDATA%\FieldCure\Mcp.Outbox\sessions\

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