Skip to content

Basic Installation

Building a Simple Mailserver


Adding the docker network's gateway to the list of trusted hosts, e.g. using the network or connected-networks option, can create an open relay, for instance if IPv6 is enabled on the host machine but not in Docker.

We are going to use this docker based mailserver:

  • First create a directory for the mailserver and get the setup script:

    mkdir -p /var/ds/
    cd /var/ds/
    curl -o \
    chmod a+x ./
  • Create the file docker-compose.yml with a content like this:


    version: '2'
        image: mailserver/docker-mailserver:latest
        hostname: mail
        container_name: mail
        - "25:25"
        - "587:587"
        - "465:465"
        - ./data/:/var/mail/
        - ./state/:/var/mail-state/
        - ./config/:/tmp/docker-mailserver/
        - /var/ds/wsproxy/letsencrypt/:/etc/letsencrypt/
        - PERMIT_DOCKER=network
        - SSL_TYPE=letsencrypt
        - ONE_DIR=1
        - DMS_DEBUG=1
        - ENABLE_CLAMAV=0
        - ENABLE_FAIL2BAN=1
        - NET_ADMIN
        - SYS_PTRACE

    For more details about the environment variables that can be used, and their meaning and possible values, check also these:

    Make sure to set the proper domainname that you will use for the emails. We forward only SMTP ports (not POP3 and IMAP) because we are not interested in accessing the mailserver directly (from a client). We also use these settings:

    • PERMIT_DOCKER=network because we want to send emails from other docker containers.
    • SSL_TYPE=letsencrypt because we will manage SSL certificates with letsencrypt.
  • We need to open ports 25, 587 and 465 on the firewall:

    ufw allow 25
    ufw allow 587
    ufw allow 465

    On your server you may have to do it differently.

  • Pull the docker image: docker pull mailserver/docker-mailserver:latest

  • Now generate the DKIM keys with ./ config dkim and copy the content of the file config/opendkim/keys/domain.tld/mail.txt on the domain zone configuration at the DNS server. I use bind9 for managing my domains, so I just paste it on

    mail._domainkey IN      TXT     ( "v=DKIM1; h=sha256; k=rsa; "
            "iqq3bD/BVlwKRp5gH6TEYEmx8EBJUuDxrJhkWRUk2VDl1fqhVBy8A9O7Ah+85nMrlOHIFsTaYo9o6+cDJ6t1i6G1gu+bZD0d3/3bqGLPBQV9LyEL1Rona5V7TJBGg099NQkTz1IwIDAQAB" )  ; ----- DKIM key mail for
  • Add these configurations as well on the same file on the DNS server:

    mail      IN  A
    ; mailservers for
        3600  IN  MX  1
    ; Add SPF record
              IN TXT "v=spf1 mx ~all"

    Then don't forget to change the serial number and to restart the service.

  • Get an SSL certificate from letsencrypt. I use wsproxy for managing SSL letsencrypt certificates of my domains:

    cd /var/ds/wsproxy
    ds domains-add mail
    ds get-ssl-cert --test
    ds get-ssl-cert

    Now the certificates will be available on /var/ds/wsproxy/letsencrypt/live/

  • Start the mailserver and check for any errors:

    apt install docker-compose
    docker-compose up mail
  • Create email accounts and aliases with SPOOF_PROTECTION=0:

    ./ email add passwd123
    ./ email add passwd123
    ./ alias add
    ./ alias add
    ./ email list
    ./ alias list

    Aliases make sure that any email that comes to these accounts is forwarded to my real email address, so that I don't need to use POP3/IMAP in order to get these messages. Also no anti-spam and anti-virus software is needed, making the mailserver lighter.

  • Or create email accounts and aliases with SPOOF_PROTECTION=1:

    ./ email add passwd123
    ./ email add passwd123
    ./ alias add
    ./ alias add
    ./ alias add
    ./ alias add
    ./ email list
    ./ alias list

    This extra step is required to avoid the 553 5.7.1 Sender address rejected: not owned by user error (the account used for setting up gmail is and )

  • Send some test emails to these addresses and make other tests. Then stop the container with ctrl+c and start it again as a daemon: docker-compose up -d mail.

  • Now save on Moodle configuration the SMTP settings and test by trying to send some messages to other users:

    • SMTP hosts:
    • SMTP security: SSL
    • SMTP username:
    • SMTP password: passwd123