SMTP Duplicator (gosmtp-dup)

An SMTP relay server that duplicates incoming emails to multiple destinations - one primary server and multiple backup servers.

Overview

This Go application acts as an SMTP proxy that receives emails and forwards them to:

• Primary destination: Critical delivery - if this fails, the original email sending fails
• Backup destinations: Additional copies sent asynchronously for redundancy

The main purpose of this program is to enable smooth migration from one mail server to another (or multiple servers) by duplicating traffic in real-time. This allows you to gradually transition your mail infrastructure while ensuring no emails are lost during the migration process.

Architecture

graph TD
    A[Email Client] -->|SMTP| B[Postfix Server]
    B -->|Transport Map| C[GoSMTP-dup :2525]

    C -->|Synchronous| D[Primary Mail Server]
    C -->|Asynchronous| E[Backup Mail Server 1]
    C -->|Asynchronous| F[Backup Mail Server 2]

    D -->|Success/Failure| C
    E -.->|Background| C
    F -.->|Background| C

    C -->|Response| B
    B -->|Response| A

    style C fill:#e1f5fe
    style D fill:#c8e6c9
    style E fill:#fff3e0
    style F fill:#fff3e0
    style B fill:#f3e5f5

    classDef primary stroke:#4caf50,stroke-width:3px
    classDef backup stroke:#ff9800,stroke-width:2px,stroke-dasharray: 5 5
    classDef proxy stroke:#2196f3,stroke-width:3px

    class D primary
    class E,F backup
    class C proxy

Loading

Flow:

1. Email clients send to Postfix
2. Postfix routes via transport maps to GoSMTP-dup
3. GoSMTP-dup sends synchronously to primary server (must succeed)
4. GoSMTP-dup sends asynchronously to backup servers (failures logged only)
5. Response based on primary server result

Features

• ✅ Dual delivery mode: Synchronous primary, asynchronous backups
• ✅ High availability: Email redundancy across multiple servers
• ✅ Environment variables: Configure via env vars or YAML
• ✅ Docker ready: Multi-architecture containers
• ✅ Postfix integration: Easy integration with existing mail servers
• ✅ Structured logging: Comprehensive logging with Zap

Documentation

📖 Complete documentation available at: https://cabonemailserver.github.io/GoSMTP-dup/

Quick Links

Topic	Description
Installation	Binary, Docker, and source installation
Configuration	YAML configuration options and examples
Environment Variables	Complete env var configuration guide
Docker Usage	Docker, Docker Compose, and Kubernetes
Postfix Integration	Step-by-step Postfix integration

Quick Start

Docker (Recommended)

docker run -d \
  --name smtp-duplicator \
  -p 2525:2525 \
  -e SMTP_DUP_SMTP_LISTEN="0.0.0.0:2525" \
  -e SMTP_DUP_RELAY_DESTINATION_PRIMARY="primary.example.com:25" \
  -e SMTP_DUP_RELAY_DESTINATION_BACKUPS="backup1.example.com:25,backup2.example.com:25" \
  ghcr.io/cabonemailserver/gosmtp-dup:latest

Binary

# Download for Linux
wget https://github.com/CaboneMailServer/GoSMTP-dup/releases/latest/download/gosmtp-dup-linux-amd64
chmod +x gosmtp-dup-linux-amd64
./gosmtp-dup-linux-amd64

Basic Configuration

Create a config.yaml file or use environment variables:

YAML Config

smtp:
  listen: "0.0.0.0:2525"
  domain: "mail.example.com"
relay:
  destination_primary: "primary.example.com:25"
  destination_backups:
    - "backup1.example.com:25"
    - "backup2.example.com:25"

Environment Variables

export SMTP_DUP_SMTP_LISTEN="0.0.0.0:2525"
export SMTP_DUP_RELAY_DESTINATION_PRIMARY="primary.example.com:25"
export SMTP_DUP_RELAY_DESTINATION_BACKUPS="backup1.example.com:25,backup2.example.com:25"

📚 For detailed configuration options, migration scenarios, and advanced setups, see the complete documentation

Use Cases

• 🔄 Mail server migration: Gradually migrate with real-time duplication
• 🔒 Email backup: Ensure delivery to multiple servers
• 📊 Load distribution: Distribute email load across servers
• 🚨 Disaster recovery: Maintain backup mail servers

Downloads

Platform	Download
Linux AMD64	Download
Linux ARM64	Download
Windows AMD64	Download
macOS AMD64	Download
macOS ARM64	Download

Docker Images

• Registry: ghcr.io/cabonemailserver/gosmtp-dup
• Latest: ghcr.io/cabonemailserver/gosmtp-dup:latest

License

MIT License - see LICENSE.md file for details.

Contributing

Contributions are welcome! Please open issues and pull requests on GitHub.