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imixs/exim4

This Docker image provides a mail transfer agent (MTA) running as a smarthost for Docker containers. The container can be used to send out e-mails from other containers.

The MTA is based on Exim4. The Image was inspired by the Docker Image from greinacker/exim4. The Docker image is based on debian:jessie.

Features

  • inherit form debian:jessie
  • provide a minimal MTA Smarthost Configuration

Environment

imixs/exim4 provides the following environment variables

  • EXIM_SMARTHOST - your target mail server
  • EXIM_PASSWORD - authenticating to a remote host as a client.
  • EXIM_ALLOWED_SENDERS - allowed sender IP/Network addresses (default=172.17.0.0/24:127.0.0.1)
  • EXIM_MESSAGE_SIZE_LIMIT - overwrites the default message_size_limit of 50m

1. Install Docker

Follow the Docker installation instructions for your host system.

2. Running and stopping a container

The container can be started in background as an demon. You can start an instance with the command:

docker run --name="smarthost" -d \
-e EXIM_SMARTHOST="target.mail.server.example:25" \
-e EXIM_PASSWORD="target.mail.server.example:login:password" \
imixs/exim4

The environment parameter 'EXIM_SMARTHOST' points to the target mail sever used by exim4. The environment parameter 'EXIM_PASSWORD' is written to the exim4/passwd file. This parameter contains the target mail server, user and login data.

To stop and remove the Docker container run the Docker command:

docker stop smarthost && docker rm smarthost

3. How to Deploy the Service

The imixs/exim4 service is supposed to be run as part of a docker service stack. This means that the service is included in a docker-compose.yml file which already contains all the environment variables as shown in the first part of this readme.

The following example shows a service definition of the exim4 service ready to act as smarthost for Office365.

...
  smarthost:
    image: imixs/exim4:latest
    environment:
      EXIM_SMARTHOST: "smtp.office365.com:587"
      EXIM_PASSWORD: "*.office.com:sender@mycompany.com:mypassword"
      EXIM_MESSAGE_SIZE_LIMIT: "1M"
      EXIM_ALLOWED_SENDERS: "10.1.7.0/24"
   
  	....

Please note that for Office365 it must be configured all the Microsoft DNS wildcard.

The MESSAGE_SIZE_LIMIT

Exim4 has a default MESSAGE_SIZE_LIMIT of 50M. Exim4 will reject emails with larger size to avoid spam mail. You can set different size here if you provide the environment variable named 'EXIM_MESSAGE_SIZE_LIMIT'.

Example:

-e EXIM_MESSAGE_SIZE_LIMIT=100M

Note: The value must be set as an integer and may not be surrounded in quotes. (100M = 100 MB, where 104857600 is the same value in bytes)

Within your imixs/exim4 docker container you can verify the current setting with the command:

$ exim4 -bP message_size_limit

3. Testing

To follow the exim4 server log:

docker logs -f smarthost

To test your smarthost configuration, first log into the bash of your smarthost:

# log into bash
docker exec -it smarthost /bin/bash

With the following command you can test sending out an email

echo "This is the message" | mail -s "The subject" captain.kirk@myhost.com -aFrom:sender@myhost.com

4. The Exim4 Mail Queue

In Exim4 mails are queued under the spooling directory:

/var/spool/exim4

To test if mails exist in the queue run:

$ mailq

You can force a retry to send queued mails with:

/usr/sbin/exim_tidydb -t 1d /var/spool/exim4 retry > /dev/null

Linking the Container

You can link the smarthost Docker container to other containers in your Docker network to allow them to send out mail via the smarthost.

Take care about the default environment setting:

EXIM_ALLOWED_SENDERS=172.17.0.0/24:127.0.0.1

This environment variable defines, if other docker containers running on your docker host, are allowed to use the MTA for sending out e-mails. The default setting is "172.17.0.0/24:127.0.0.1", which allows all linked containers to send mails. Customize this parameter if you have custom needs in your docker virtual network. If the setting is not set correct, you will typically see a log message like

...exim4 IP address .. relay not permitted...

Kubernetes

In the yaml file kubernetes.yaml you can find an example how to deploy the container as a StatefulSet into a Kubernetes cluster. In this example an external data volume for the exim4 spool database is added. You may customize this example to your own needs.

Contribute

The source is available on Github. Please report any issues.

To build the image from the Dockerfile run:

docker build --tag=imixs/exim4 .

Push manually to Docker repo (Docker-Hub)

To push the image to a docker repo:

docker build -t imixs/exim4:X.X.X .

docker push imixs/exim4:X.X.X