If your applications need to send emails reliably, an SMTP relay is one of the cleanest solutions.

In this tutorial, we will build a lightweight SMTP relay using Docker and Postfix on Ubuntu. Your applications will send email locally to the relay, and the relay will securely forward mail through providers like Amazon SES, SendGrid, Mailgun, or Gmail SMTP.

This setup is ideal for:

Laravel applications
WordPress websites
Node.js apps
Dockerized services
Internal notification systems
Transactional emails

Architecture

Application
    ↓ SMTP
Docker Postfix Relay
    ↓ TLS SMTP
Amazon SES / SendGrid / Mailgun
    ↓
Recipient Inbox

Prerequisites

Before starting, make sure you have:

Ubuntu server
Docker installed
Docker Compose plugin installed
SMTP provider credentials

Supported providers include:

Amazon SES
SendGrid
Mailgun
Gmail SMTP
Postmark

Step 1 — Install Docker

Update Ubuntu:

sudo apt update

Install Docker:

sudo apt install -y docker.io docker-compose-plugin

Enable Docker:

sudo systemctl enable --now docker

Verify installation:

docker --version

Optional: run Docker without sudo

sudo usermod -aG docker $USER
newgrp docker

Step 2 — Create Project Directory

Create a working directory:

sudo mkdir -p /opt/smtp-relay
cd /opt/smtp-relay

Step 3 — Create Persistent Storage

Create directories for mail queue and logs:

sudo mkdir -p relay
sudo mkdir -p logs

These directories ensure queued emails survive container restarts.

Step 4 — Create Docker Compose File

Create a docker-compose.yml file:


services:
  smtp-relay:
    image: boky/postfix
    container_name: smtp-relay
    restart: unless-stopped

    ports:
      - "25:25"

    environment:
      # Upstream SMTP provider
      RELAYHOST: smtp.gmail.com
      RELAYHOST_PORT: 587
      RELAYHOST_USERNAME: YourSMTPEnabledGmailUserID
      RELAYHOST_PASSWORD: YourGmailPassword

      # Allowed sender domains
      ALLOWED_SENDER_DOMAINS: wempro.com,pumpsandinstrumentations.com

      # Relay hostname
      POSTFIX_myhostname: relay.vmi3202307.local
      POSTFIX_mynetworks: 127.0.0.0/8 172.16.0.0/12 192.168.0.0/16

      POSTFIX_smtpd_recipient_restrictions: permit_mynetworks,reject_unauth_destination

      TZ: UTC

    volumes:
      # Mail queue persistence
      - ./relay:/var/spool/postfix

      # Optional logs
      - ./logs:/var/log

    logging:
      driver: json-file
      options:
        max-size: "10m"
        max-file: "3"

Save the file.

Step 5 — Start the SMTP Relay

Launch the container:

docker compose up -d

Verify container status:

docker ps

View logs:

docker logs -f smtp-relay

Step 6 — Test Email Sending

Install swaks:

sudo apt install -y swaks

Send a test email:

swaks \
  --to you@example.com \
  --from noreply@yourdomain.com \
  --server localhost:25 \
  --header "Subject: SMTP Relay Test" \
  --body "SMTP relay is working"

Successful output:

250 2.0.0 Ok: queued as ...

Step 7 — Configure Your Application

Your applications should connect to:

localhost:25

Example DSN:

smtp://localhost:25

Laravel .env example:

MAIL_MAILER=smtp
MAIL_HOST=localhost
MAIL_PORT=25
MAIL_USERNAME=null
MAIL_PASSWORD=null
MAIL_ENCRYPTION=null
MAIL_FROM_ADDRESS=noreply@yourdomain.com
MAIL_FROM_NAME="Your App"

Multiple Domain Support

To allow multiple sender domains:

ALLOWED_SENDER_DOMAINS: domain1.com,domain2.com,domain3.com

Why Use an SMTP Relay?

Benefits include:

centralized email handling
provider abstraction
email queueing
retry handling
cleaner application configuration
rate limiting
easier provider switching

Important Security Tips

Do NOT Create an Open Relay

Never use:

POSTFIX_mynetworks: 0.0.0.0/0

This will allow the internet to abuse your server for spam.

SPF, DKIM, and Deliverability

For production use, verify your domain with your SMTP provider and configure:

SPF
DKIM
DMARC

Without these, emails may land in spam folders.

Queue Management

View mail queue:

docker exec -it smtp-relay postqueue -p

Flush queue:

docker exec -it smtp-relay postqueue -f

Final Thoughts

A Dockerized SMTP relay is a lightweight and reliable solution for modern applications. By combining Postfix with providers like Amazon SES or SendGrid, you get:

reliable delivery
secure outbound SMTP
local application integration
retry and queue management
simplified infrastructure

This setup works especially well for Docker-based deployments and internal application stacks.

Happy emailing!

Tag: relay

How to Build a Docker SMTP Relay on Ubuntu Using Postfix