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Setting Up a Secure Mail Server with Docker Mailserver

@red_sh4d0w / August 10, 2024
4 min read

Setting Up a Secure Mail Server with Docker Mailserver

Github Repo Link

In today's digital world, managing a reliable and secure mail server is crucial for any organization. With the rise of containerization technologies like Docker, setting up and managing a mail server has become more accessible and streamlined. In this article, we will walk through the process of setting up a mail server using Docker Mailserver, covering prerequisites, installation, configuration, and best practices to ensure a secure and efficient email system.

Introduction

Docker Mailserver is a popular open-source mail server suite that simplifies the deployment and management of a mail server using Docker containers. It provides an all-in-one solution for handling SMTP, IMAP, and POP3 services, along with support for security features such as DKIM, SPF, and DMARC. This article will guide you through the complete setup of Docker Mailserver, from initial installation to fine-tuning your server for optimal performance and security.

Prerequisites

Before diving into the setup process, ensure you have the following prerequisites:

  • Mail Domain Name: A registered domain name for your mail server (e.g., mail.example.com).
  • Docker and Docker Compose: Installed on your server.

Setup

1. Create Docker Compose File

Create a docker-compose.yml file with the following content to define the mail server service:

version: "3"

networks:
  mailserver:
    name: mailserver

services:
  mailserver:
    image: docker.io/mailserver/docker-mailserver:13.0.1
    container_name: mail.example.com
    hostname: mail.example.com
    env_file: mailserver.env
    ports:
      - "25:25"    # SMTP
      - "143:143"  # IMAP4
      - "465:465"  # ESMTP
      - "587:587"  # ESMTP
      - "993:993"  # IMAP4
    volumes:
      - ./docker-data/dms/mail-data/:/var/mail/
      - ./docker-data/dms/mail-state/:/var/mail-state/
      - ./docker-data/dms/mail-logs/:/var/log/mail/
      - ./docker-data/dms/config/:/tmp/docker-mailserver/
      - /etc/localtime:/etc/localtime:ro
      - /home/docker-user/traefik/acme.json:/etc/letsencrypt/acme.json:ro
    restart: always
    stop_grace_period: 1m
    cap_add:
      - NET_ADMIN
    healthcheck:
      test: "ss --listening --tcp | grep -P 'LISTEN.+:smtp' || exit 1"
      timeout: 3s
      retries: 5
    networks:
      - mailserver

2. Environment File

Adjust any of the environment variables to better fit your configuration and needs.

3. Create Directories

Create the necessary directories for storing mail data, state, logs, and configuration:

mkdir -p docker-data/dms/mail-data
mkdir -p docker-data/dms/mail-state
mkdir -p docker-data/dms/mail-logs
mkdir -p docker-data/dms/config

4. Run Docker Compose

Start the mail server container:

docker-compose up -d

5. Create Email Address

Add an email address using the provided setup.sh script:

./setup.sh email add admin@mail.example.com

6. Configure DKIM

Generate DKIM keys and configure your DNS:

./setup.sh config dkim

Add the DKIM TXT record to your DNS settings based on the output file located at ./docker-data/dms/config/opendkim/keys/mail.example.com/mail.txt.

7. Restart the Container

After configuring DKIM and SPF records, restart the container to apply changes:

docker-compose restart

DNS Records

1. MX Record

  • Name: @
  • Type: MX
  • Value: mail.example.com
  • Priority: 1

2. DKIM Record

  • Name: mail._domainkey.mail
  • Type: TXT
  • Value: 
    v=DKIM1; h=sha256; k=rsa; p=MIIBIRANDOMudT8ywZatm/vF1Bofy3A+5iPhiIPJFmztU5mQuWcnu4Ml5WYQRP3UFyRANDOMUxNwqZfHsZ+Xl+uac+6AeUduDtRQSxtpDauNM4X/f7sGPlu3ExtoRANDOMrmVAM5NIPrjD1dhUcEYKaaCtbZ+OxZ688lsECgHa7dk1Zchv+8RpKO2nRvB2pUg/5Cls9eATVkkHh4qY0/cWPt8PmaBEtANgAPXeJxDz3Q/X4vS5W+2hsRANDOMcBxGBfo7VPLAnL87GDzG3HUONSYn0Ct6YJRANDOMAlYswiSuZQwIDAQAB
    Use the generated DKIM key

3. SPF Record

  • Name: @
  • Type: TXT
  • Value: 
    v=spf1 mx a:mail.example.com include:mail.example.com +all

4. DMARC Record (for completeness)

  • Name: _dmarc
  • Type: TXT
  • Value: 
    v=DMARC1; p=none; rua=mailto:admin@mail.example.com; ruf=mailto:admin@mail.example.com; sp=none; ri=86400

Testing and Troubleshooting

Postfix Email Reception Troubleshooting Guide

If you are experiencing issues with receiving emails, follow these detailed guidelines to troubleshoot and resolve the problem:

1. Verify Configuration

  • Check Configuration Files:

    • Ensure the myhostname and mydomain variables are correctly set.
    • Refer to the provided utils/postfix.conf.example Example File Linkfor configuration details. Check the IMPORTANT section. Verify that these values are correctly set in the /etc/postfix/main.cf file. 
      mydestination = localhost.$mydomain, localhost
myhostname = mail.example.com
mydomain = mail.example.com

  • Apply Changes:

    • Reload Postfix to apply if there are any changes in the config file. 
      postfix reload

2. Check Logs

  • Review Logs:
    • Examine Postfix logs for any errors or warnings that might indicate configuration issues or other problems.

3. Network and Firewall

  • Check Port Accessibility:
    • Ensure that necessary ports (e.g., 25 for SMTP, 587 for submission) are open and not blocked by firewalls.

4. Testing

  • Verify Connectivity:
    • Use tools like telnet or nc to test connectivity to the mail server.

5. Additional Tips

1. Email Delivery Testing

Use online tools to test the configuration of your mail server. Some useful tools include:

2. Logs and Debugging

Monitor the logs to troubleshoot issues:

docker-compose logs -f

Check logs located in docker-data/dms/mail-logs/ for detailed information.

3. Verify DNS Records

Ensure that DKIM, SPF, and MX records are correctly configured using tools like:

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