Self-hosted multi-tenant SMTP relay with web dashboard. Routes outbound email through multiple upstream SMTP providers based on sender address. Built with FastAPI, Postfix, PostgreSQL, Docker.

Demo

Try it at relay.mailtoinbox.vip - register a free account and test the full system.

Configuration (.env)

Required

Database (PostgreSQL)

Default values work out of the box. Change passwords for production.

System SMTP (optional)

Used for sending verification and password reset emails to users. If left empty, these emails are logged to console only (dev mode).

Ports

Remap in docker-compose.yml if needed.

Quick Start

git clone https://github.com/toinbox/simplerelay.git
cd simplerelay
cp env.example .env
nano .env
docker compose up --build -d

Dashboard: http://your-server:8080 SMTP relay endpoint: your-server:2525

User Panel

Every registered user gets access to:

Dashboard - overview of today's sent count, errors, provider health status with response times, and recent mail log entries.

Setup Wizard - guided first-time setup: enter email, auto-detect provider and SMTP settings, test connection, run DNS check (SPF/DKIM/DMARC), configure access control (IP whitelist, optionally SMTP AUTH), send test email.

Providers - manage SMTP providers (upstream accounts used for sending). Each provider has:

• Auto-detection of SMTP settings from email address (Gmail, Outlook, Yahoo, Seznam, Zoho, SES, SendGrid, Mailgun, or custom SMTP)
• App password guidance for providers that require it (Gmail, Outlook, Yahoo) with direct links
• SMTP connection test
• DNS validation (SPF, DKIM, DMARC) with provider-aware DKIM selector scanning
• Send test email through the relay
• Daily sending limit with usage counter
• Per-provider access control (required) - IP whitelist is mandatory for the relay to accept connections. SMTP AUTH credentials can be added as additional authentication. Supports password generation, show/hide toggle, and regeneration.

Mail Logs - searchable log of all sent, failed, and bounced messages with sender, recipient, subject, status, client IP, and timestamp.

Admin Panel

Admin users see additional sections:

User Management - list all registered users, activate/deactivate accounts (deactivation auto-suspends all user's relays), change user roles (user/admin), set maximum number of relays per user, set relay expiry period (auto-expire after N days), delete users with cascade.

Proxy Management - manage outbound IPs and SOCKS5/HTTP proxies. Each proxy entry has:

• Protocol selection (direct IP, SOCKS5, HTTP)
• Provider type filtering (assign specific proxies to specific provider types, e.g. Gmail proxies vs. catch-all)
• Round-robin assignment with load tracking
• Proxy testing (HTTP connectivity + SMTP handshake through proxy)
• Enable/disable toggle

Provider Type Limits - set global daily sending limits per provider type (Gmail, Outlook, Seznam, custom, etc.) applied across all users.

Admin Dashboard - global stats: total/active users, total/active/locked relays, total/active proxies, sent and errors today.

Provider Oversight - view any user's relay list, lock/unlock individual providers with reason.

System Features

• Postfix-based relay with per-sender routing to correct upstream SMTP
• Built-in policy server (access control) - every provider requires a whitelisted IP address. Without it, the relay rejects all connections. SMTP AUTH credentials are optional as additional layer. No open relay possible.
• Automatic provider detection from email address (MX/DNS lookup fallback for unknown domains)
• User registration with email verification, password reset, password change
• JWT authentication with httponly cookies
• Relay expiry - providers auto-expire after admin-configured number of days
• Health monitoring - periodic SMTP connection checks with status tracking
• Multilingual UI - English, Czech, German, Russian, Spanish with runtime language switching
• Single Docker Compose deployment (Postfix + PostgreSQL + FastAPI + React/Vite)

Home Network / Homelab Use Case

SimpleRelay works great as a central SMTP relay on your home network. Instead of configuring Gmail credentials in every device separately, you point everything at SimpleRelay on your LAN and manage credentials in one place.

Your home network (192.168.1.x)
┌─────────────────────────────────────────────────────────┐
│                                                         │
│  Proxmox ──────┐                                        │
│  Synology NAS ─┤                                        │
│  pfSense ──────┤    SimpleRelay          Gmail/Outlook   │
│  Grafana ──────┼──► 192.168.1.50:2525 ──► SMTP ──► ✉️  │
│  Uptime Kuma ──┤    (no auth needed)     (app password)  │
│  Home Asst. ───┤                                        │
│  Cameras ──────┘                                        │
│                                                         │
└─────────────────────────────────────────────────────────┘
  No NAT. No port forwarding. No public IP needed.

• 10 devices = 10 places to maintain Gmail app passwords. With SimpleRelay: one.
• No need for public IP, port forwarding, or your own mail server.
• Add 0.0.0.0/0 as allowed subnet and every device on your network can send.

See COMPATIBILITY.md for the full list of 50+ compatible devices with setup instructions - servers, firewalls, cameras, monitoring tools, backup software, UPS, printers and more.

Tech Stack

FastAPI (Python 3.12), Postfix, PostgreSQL 16, React + Vite, Docker Compose

License

MIT