Mail Relay for Docker Containers

This is a simple container running postfix. It will accept mails and relay them over an authenticated smarthost to their destination.

Mails will be accepted on Port 25 for delivery without authentication by the containers. Access control is meant to happen on the network level, i.e. by limiting which hosts/containers can connect to the relay and whitelisting network addresses.

Building

Building the container will fetch debian:bullseye-slim as the container's base.

$ docker-compose build

Configuration

Edit env.sample and save it as env.

• mydomain: Domain part of outgoing mail addresses
• relayhost: fqdn for your relay host
• relayuser: username for authentication at the remote mail relay
• relaypassword: password for authentication at the remote mail relay

Optional configuration:

• myhostname: Hostname the docker container should assume for itself. Defaults to mail.
• mynetworks: Networks (CIDR) that are whitelisted for mail delivery. Defaults to 192.168.0.0/16,172.16.0.0/12.
• relayport: Port the remote mail relay is reachable at. Defaults to 587 (submission)

Running

Start the container:

$ docker-compose up -d

Watch logfiles:

$ docker-compose logs -f

App configuration

Make sure your apps' docker-compose.yaml have the mail network configured for all services that should be able to send mails, e.g.:

---
version: "3.5"
services:
  busybox:
    image: busybox
  networks:
    - mail
networks:
  mail:
    external: true
    name: mail

Pay attention to the networks settings (both of them).

Then, configure your apps to the following settings:

• SMTP Host: mail_postfix_1
• SMTP Port: 25
• SMTP Auth: none at all
• StartTLS/SSL: Only StartTLS will work on port 25. It's possible, but not required when all containers run on the same machine and communication doesn't go over the wire.
• Certificate validation: off. We won't generate valid certificates for mail_postfix_1.

Copyright

Florian Heinle github@florianheinle.de

MIT-License (see LICENSE)