docker-mailserver/demo-setups/relay-compose.yaml

# Docs: https://docker-mailserver.github.io/docker-mailserver/v14.0/config/advanced/mail-forwarding/relay-hosts/
# Additional context, with CLI commands for verification:
# https://github.com/docker-mailserver/docker-mailserver/issues/4136#issuecomment-2253693490

services:
  # This would represent your actual DMS container:
  dms-sender:
    image: mailserver/docker-mailserver:latest # :14.0
    hostname: mail.example.test
    environment:
      # All outbound mail will be relayed through this host
      # (change the port to 587 if you do not want the postfix-main.cf override)
      - DEFAULT_RELAY_HOST=[smtp.relay-service.test]:465
      # Your relay host credentials.
      # (since the relay in the example is DMS, the relay account username is a full email address)
      - RELAY_USER=relay-user@relay-service.test
      - RELAY_PASSWORD=secret
      # The mail client (swaks) needs to connect with TLS:
      - SSL_TYPE=manual
      - SSL_KEY_PATH=/tmp/tls/key.pem
      - SSL_CERT_PATH=/tmp/tls/cert.pem
    # You would usually have `volumes` instead of this `configs`:
    configs:
      - source: dms-main
        target: /tmp/docker-mailserver/postfix-main.cf
      - source: dms-accounts
        target: /tmp/docker-mailserver/postfix-accounts.cf
      # Authenticating on port 587 or 465 enforces TLS requirement:
      - source: tls-cert
        target: /tmp/tls/cert.pem
      - source: tls-key
        target: /tmp/tls/key.pem
      # This is only needed if you want to verify the TLS cert chain with swaks
      # (normally with public CA providers like LetsEncrypt this file is already available to a mail client)
      - source: tls-ca-cert
        target: /tmp/tls/ca-cert.pem

  # Pretend this is your third-party relay service:
  dms-relay:
    image: mailserver/docker-mailserver:latest # :14.0
    hostname: smtp.relay-service.test
    environment:
      # WORKAROUND: Bypass security checks from the mail-client (dms-sender container)
      # (avoids needing valid DNS for this example)
      - PERMIT_DOCKER=connected-networks
      # TLS is required when relaying to dms-relay via ports 587 / 465
      # (dms-relay will then relay the mail to dms-destination over port 25)
      - SSL_TYPE=manual
      - SSL_KEY_PATH=/tmp/tls/key.pem
      - SSL_CERT_PATH=/tmp/tls/cert.pem
    # WORKAROUND: `links` is required due to lack of properly configured DNS.
    # (resolves destination.test to the IP of the dms-destination container)
    links:
      - "dms-destination:destination.test"
    configs:
      - source: dms-accounts-relay
        target: /tmp/docker-mailserver/postfix-accounts.cf
      - source: tls-cert
        target: /tmp/tls/cert.pem
      - source: tls-key
        target: /tmp/tls/key.pem

  # Pretend this is another mail server that your target recipient belongs to (like Gmail):
  dms-destination:
    image: mailserver/docker-mailserver:latest # :14.0
    hostname: mail.destination.test
    # Same workaround for purposes of the example, with the target recipient provisioned to accept mail
    environment:
      - PERMIT_DOCKER=connected-networks
    configs:
      - source: dms-accounts-destination
        target: /tmp/docker-mailserver/postfix-accounts.cf

# Using the Docker Compose `configs.content` feature instead of volume mounting separate files.
# NOTE: This feature requires Docker Compose v2.23.1 (Nov 2023) or newer:
# https://github.com/compose-spec/compose-spec/pull/446
configs:
  # DMS expects an account to be configured to run, this example provides accounts already created.
  # Login credentials:
  # user: "john.doe@example.test" password: "secret"
  # user: "relay-user@relay-service.test" password: "secret"
  # user: "jane.doe@destination.test" password: "secret"
  dms-accounts:
    # NOTE: `$` needed to be repeated to escape it,
    # which opts out of the `compose.yaml` variable interpolation feature.
    content: |
      john.doe@example.test|{SHA512-CRYPT}$$6$$sbgFRCmQ.KWS5ryb$$EsWrlYosiadgdUOxCBHY0DQ3qFbeudDhNMqHs6jZt.8gmxUwiLVy738knqkHD4zj4amkb296HFqQ3yDq4UXt8.      

  dms-accounts-relay:
    content: |
      relay-user@relay-service.test|{SHA512-CRYPT}$$6$$o65y1ZXC4ooOPLwZ$$7TF1nYowEtNJpH6BwJBgdj2pPAxaCvhIKQA6ww5zdHm/AA7aemY9eoHC91DOgYNaKj1HLxSeWNDdvrp6mbtUY.      

  dms-accounts-destination:
    content: |
      jane.doe@destination.test|{SHA512-CRYPT}$$6$$o65y1ZXC4ooOPLwZ$$7TF1nYowEtNJpH6BwJBgdj2pPAxaCvhIKQA6ww5zdHm/AA7aemY9eoHC91DOgYNaKj1HLxSeWNDdvrp6mbtUY.      

  # This is `postfix-main.cf`, single line change to make all outbound SMTP connections over port 465 instead of 25 (default)
  # If you selectively relay mail, you would need to adjust this on the relay service in `/etc/postfix/master.cf`,
  # However DMS presently modifies this when using the DMS Relay Host feature support, which may override `postfix-master.cf` or `user-patches.sh` due to `check-for-changes.sh`.
  dms-main:
    content: |
      smtp_tls_wrappermode=yes      

  # TLS files:
  # - Use an ECDSA cert that's been signed by a self-signed CA for TLS cert verification.
  # - This cert is only valid for mail.example.test, mail.destination.test, smtp.relay-service.test

  # `swaks` run in the container will need to reference this CA cert file for successful verficiation (optional).
  tls-ca-cert:
    content: |
      -----BEGIN CERTIFICATE-----
      MIIBfTCCASKgAwIBAgIRAMAZttlRlkcuSun0yV0z4RwwCgYIKoZIzj0EAwIwHDEa
      MBgGA1UEAxMRU21hbGxzdGVwIFJvb3QgQ0EwHhcNMjEwMTAxMDAwMDAwWhcNMzEw
      MTAxMDAwMDAwWjAcMRowGAYDVQQDExFTbWFsbHN0ZXAgUm9vdCBDQTBZMBMGByqG
      SM49AgEGCCqGSM49AwEHA0IABJX2hCtoK3+bM5I3rmyApXLJ1gOcVhtoSSwM8XXR
      SEl25Kkc0n6mINuMK8UrBkiBUgexf6CYayx3xVr9TmMkg4KjRTBDMA4GA1UdDwEB
      /wQEAwIBBjASBgNVHRMBAf8ECDAGAQH/AgEBMB0GA1UdDgQWBBQD8sBrApbyYyqU
      y+/TlwGynx2V5jAKBggqhkjOPQQDAgNJADBGAiEAi8N2eOETI+6hY3+G+kzNMd3K
      Sd3Ke8b++/nlwr5Fb/sCIQDYAjpKp/MpTDWICeHC2tcB5ptxoTdWkTBuG4rKcktA
      0w==
      -----END CERTIFICATE-----      

  tls-key:
    content: |
      -----BEGIN EC PRIVATE KEY-----
      MHcCAQEEIOc6wqZmSDmT336K4O26dMk1RCVc0+cmnsO2eK4P5K5yoAoGCCqGSM49
      AwEHoUQDQgAEFOWNgekKKvUZE89vJ7henUYxODYIvCiHitRc2ylwttjqt1KUY1cp
      q3jof2fhURHfBUH3dHPXLHig5V9Jw5gqeg==
      -----END EC PRIVATE KEY-----      

  tls-cert:
    content: |
      -----BEGIN CERTIFICATE-----
      MIIB9DCCAZqgAwIBAgIQE53a/y2c//YXRsz2kLm6gDAKBggqhkjOPQQDAjAcMRow
      GAYDVQQDExFTbWFsbHN0ZXAgUm9vdCBDQTAeFw0yMTAxMDEwMDAwMDBaFw0zMTAx
      MDEwMDAwMDBaMBkxFzAVBgNVBAMTDlNtYWxsc3RlcCBMZWFmMFkwEwYHKoZIzj0C
      AQYIKoZIzj0DAQcDQgAEFOWNgekKKvUZE89vJ7henUYxODYIvCiHitRc2ylwttjq
      t1KUY1cpq3jof2fhURHfBUH3dHPXLHig5V9Jw5gqeqOBwDCBvTAOBgNVHQ8BAf8E
      BAMCB4AwHQYDVR0lBBYwFAYIKwYBBQUHAwEGCCsGAQUFBwMCMB0GA1UdDgQWBBSz
      w74g+O6dcBbwienD70D8A9ESmDAfBgNVHSMEGDAWgBQD8sBrApbyYyqUy+/TlwGy
      nx2V5jBMBgNVHREERTBDghFtYWlsLmV4YW1wbGUudGVzdIIVbWFpbC5kZXN0aW5h
      dGlvbi50ZXN0ghdzbXRwLnJlbGF5LXNlcnZpY2UudGVzdDAKBggqhkjOPQQDAgNI
      ADBFAiEAoety5oClZtuBMkvlUIWRmWlyg1VIOZ544LSEbplsIhcCIHb6awMwNdXP
      m/xHjFkuwH1+UjDDRW53Ih7KZoLrQ6Cp
      -----END CERTIFICATE-----