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Mailserver behind Proxy

Using DMS behind a Proxy


If you are hiding your container behind a proxy service you might have discovered that the proxied requests from now on contain the proxy IP as the request origin. Whilst this behavior is technical correct it produces certain problems on the containers behind the proxy as they cannot distinguish the real origin of the requests anymore.

To solve this problem on TCP connections we can make use of the proxy protocol. Compared to other workarounds that exist (X-Forwarded-For which only works for HTTP requests or Tproxy that requires you to recompile your kernel) the proxy protocol:

  • It is protocol agnostic (can work with any layer 7 protocols, even when encrypted).
  • It does not require any infrastructure changes.
  • NAT-ing firewalls have no impact it.
  • It is scalable.

There is only one condition: both endpoints of the connection MUST be compatible with proxy protocol.

Luckily dovecot and postfix are both Proxy-Protocol ready softwares so it depends only on your used reverse-proxy / loadbalancer.

Configuration of the used Proxy Software

The configuration depends on the used proxy system. I will provide the configuration examples of traefik v2 using IMAP and SMTP with implicit TLS.

Feel free to add your configuration if you achieved the same goal using different proxy software below:

Traefik v2

Truncated configuration of traefik itself:

    image: # v2.5
    container_name: docker-traefik
    restart: always
      - "--providers.docker"
      - "--providers.docker.exposedbydefault=false"
      - ""
      - "--entrypoints.web.address=:80"
      - "--entryPoints.websecure.address=:443"
      - "--entryPoints.smtp.address=:25"
      - "--entryPoints.smtp-ssl.address=:465"
      - "--entryPoints.imap-ssl.address=:993"
      - "--entryPoints.sieve.address=:4190"
      - "25:25"
      - "465:465"
      - "993:993"
      - "4190:4190"

Truncated list of necessary labels on the DMS container:

    container_name: mailserver
    restart: always
      - proxy
      - "traefik.enable=true"
      - "traefik.tcp.routers.smtp.rule=HostSNI(`*`)"
      - "traefik.tcp.routers.smtp.entrypoints=smtp"
      - "traefik.tcp.routers.smtp.service=smtp"
      - ""
      - ""
      - "traefik.tcp.routers.smtp-ssl.rule=HostSNI(`*`)"
      - "traefik.tcp.routers.smtp-ssl.entrypoints=smtp-ssl"
      - "traefik.tcp.routers.smtp-ssl.tls.passthrough=true"
      - "traefik.tcp.routers.smtp-ssl.service=smtp-ssl"
      - ""
      - ""
      - "traefik.tcp.routers.imap-ssl.rule=HostSNI(`*`)"
      - "traefik.tcp.routers.imap-ssl.entrypoints=imap-ssl"
      - "traefik.tcp.routers.imap-ssl.service=imap-ssl"
      - "traefik.tcp.routers.imap-ssl.tls.passthrough=true"
      - ""
      - ""
      - "traefik.tcp.routers.sieve.rule=HostSNI(`*`)"
      - "traefik.tcp.routers.sieve.entrypoints=sieve"
      - "traefik.tcp.routers.sieve.service=sieve"
      - ""

Keep in mind that it is necessary to use port 10993 here. More information below at dovecot configuration.

Configuration of the Backend (dovecot and postfix)

The following changes can be achieved completely by adding the content to the appropriate files by using the projects function to overwrite config files.

Changes for postfix can be applied by adding the following content to docker-data/dms/config/

postscreen_upstream_proxy_protocol = haproxy

and to docker-data/dms/config/


Changes for dovecot can be applied by adding the following content to docker-data/dms/config/

haproxy_trusted_networks = <your-proxy-ip>, <optional-cidr-notation>
haproxy_timeout = 3 secs
service imap-login {
  inet_listener imaps {
    haproxy = yes
    ssl = yes
    port = 10993


Port 10993 is used here to avoid conflicts with internal systems like postscreen and amavis as they will exchange messages on the default port and obviously have a different origin then compared to the proxy.