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Postfix Relay Docker

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Postfix Gmail Relay Docker Setup

Github Repo

This repository contains a Dockerized Postfix service setup configured as a Gmail SMTP relay. The container runs Postfix with SASL authentication to send emails through Gmail’s SMTP server.

Features

  • Relays emails through Gmail’s SMTP server (smtp.gmail.com).
  • Supports authentication via Gmail’s SMTP with SASL credentials.
  • Customizable via environment variables (hostname, networks, etc.).
  • Secure connection using TLS.
  • Automatic configuration of Postfix with Docker.

Requirements

⚠️ Important: This solution only works with Gmail Workspace accounts that have SMTP relay configured. It will not work with regular Gmail accounts.

  • Docker: Ensure Docker is installed on your machine.
  • Gmail Workspace Account: This setup only works with Gmail accounts configured for relay via Google Workspace (formerly G Suite). It does not work with regular Gmail accounts. To set up Gmail as a relay, your Google Workspace admin needs to configure Gmail SMTP relay in your domain.
  • SMTP Credentials: You will need to provide your Google Workspace email and app password in the .env file.
  • Basic understanding of Postfix and email relaying.

Getting Started

1. Clone the repository

git clone https://github.com/aganet/postfix-relay-docker.git
cd postfix-relay-docker

2. Create a .env file

You need to create a .env file in the root of the repository with the following content:

SMTP_USER=your-email@gmail.com
SMTP_PASS=your-app-password
  • Replace your-email@gmail.com with your Gmail email.
  • Replace your-app-password with your Gmail App Password.

3. Build the Docker image

docker build -t postfix-relay:latest .

4. Run the Docker container

You can start the container using Docker Compose. It will automatically configure Postfix based on the environment variables defined in the .env file.

docker-compose up -d

5. Configuration

The following environment variables are used to configure the container:

Variable Description Default Value
RELAY_HOST The relay SMTP server host (usually Gmail SMTP) smtp.gmail.com
RELAY_PORT The relay SMTP server port 587
MYHOSTNAME The hostname to use for Postfix example.com
MYNETWORKS Networks allowed to relay mail 127.0.0.0/8
SASL_PASSWD SASL authentication (username:password) Required

You can override these values by setting them in the .env file or directly in the docker-compose.yml file.

6. Test Email Sending

Test the relay from another computer (replace xx with the IP of the host running this docker)

telnet xx.xx.xx.xx 25

Edit and paste the following:

EHLO testclient.com
MAIL FROM:<name@gmail.com>
RCPT TO:<name@gmail.com>
DATA
Subject: Test Email

This is a test email sent through the Postfix relay.
     
.
QUIT

Check docker logs for error messages

7. Logs

The container outputs Postfix logs to the console. You can view them with: docker logs postfix-relay or docker logs -f postfix-relay

Volumes

If you need persistent storage for Postfix queues, uncomment the volumes section in the docker-compose.yml:

volumes:
  - ./postfix-spool:/var/spool/postfix

This will mount the host directory ./postfix-spool to store the Postfix spool.

Ports

The default port exposed is 25. If you need to change it, modify the ports section in docker-compose.yml:

ports:
  - "25:25"

Troubleshooting

  • Authentication failed: Check if your Gmail Workspace account is properly configured for SMTP relay.
  • Connection refused on port 25: Ensure no other services are using port 25 on your host machine. (If postfix is already installed on the host machine could possible use port25 , disable postfix on main host machine systemctl stop postfix && systemctl disable postfix )
  • TLS/SSL errors: Verify that your SMTP settings and certificates are correct and up to date.

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