Go SMTP Relay

A minimal SMTP-SMTP Relay written in Go. The main purpose is to have a local relay which authenticates and verifies permissions for sending via a remote SMTP server (e.g. Amazon AWS, SMTP2GO).

Usage

Go

Compile the go code (e.g. go build -o smtp-relay main.go) and run the binary or use the kubernetes operator (docs coming soon...)

Docker

There are docker images avaiable on GHCR and Docker Hub

• docker.io/scheiblingco/smtp-relay:latest
• ghcr.io/scheiblingco/smtp-relay:latest

Configuration

OUT OF DATE: Needs update.

{
    "server": {
        // Hostname for the server
        "host": "smtp.example.com",

        // Port to listen on for regular/StartTLS
        "listen": ":2525",

        // Enable separate SMTPS listener
        "smtps": true,

        // Listen port for SMTPS
        "listenSmtps": ":4650",

        // Enable STARTTLS (Requires tlsCert and tlsKey)
        "startTls": true,

        // Path to TLS Certificate
        "tlsCert": "tls.crt",

        // Path to TLS Private Key
        "tlsKey": "tls.key",

        // Allow users with insecure connections (no SSL/TLS/StartTLS)
        "allowInsecure": true,

        // Timeout for read/write operations
        "readTimeout": 10,
        "writeTimeout": 10,

        // Maximum number of recipients per message
        "maxRecipients": 100,

        // Maximum message size in MB
        "maxMessageSizeMb": 30,

        // User credentials
        "credentials": {
            "all": {
                "password": "all",
                
                // Allow all domains
                "allowedDomains": []
            },
            "specific": {
                "password": "specific",

                // Allow user to send only from the specified domains
                "allowedDomains": [
                    "example.com"
                ]
            }
        }
    },
    // Remote SMTP Server Configuration
    "remote": {
        // Hostname
        "host": "mail.provider.com",

        // Port
        "port": "25",

        // Enable StartTLS
        "startTls": true,

        // Credentials
        "username": "",
        "password": "",

        // Use PLAIN authentication
        "authPlain": false,

        // Use LOGIN authentication
        "authLogin": false
    }
}

Functionality

Authentication

Authentication with username/password (PLAIN/LOGIN) defined in the json configuration.

Backend

Any SMTP backend can be used

Authorization

Set allowed sender domains for each individual account to prevent abuse.

TLS

Support for StartTLS (regular port) and SMTPS (SMTPS Port, optional).