Simple exim relay in Docker

Dockerized exim for sending mails to smarthost(s)

What is this?

I wanted a simple and small Docker container with exim in it to relay mail to a smarthost or multiple smarthosts. Applications can submit their mail to this and take advantage of exim's queueing capabilites. Also the container should be restartable without losing spooled mails.

How to use it?

$ docker build -t docker-exim .
$ docker run -it --rm \
    --init \
    --name docker-exim \
    -p "25:2525" \
    -v eximspool:/var/spool/exim \
    -e SMARTHOSTS="smtp.example.com" \
    docker-exim

This should build and start the container ready for SMTP relaying to the smarthost list.

Specify permitted subnets

If you don't specify RELAY_FROM_HOSTS then only localhost (127.0.0.1, ::1) can relay mail. Here's an example for allowing a host in the subnet 192.0.2.0/24 to relay to the smarthost:

$ docker run -it --rm \
    --init \
    --name docker-exim \
    -p "25:2525" \
    -v eximspool:/var/spool/exim \
    -e RELAY_FROM_HOSTS="<; 127.0.0.1 ; ::1 ; 192.0.2.0/24" \
    -e SMARTHOSTS="smtp.example.com" \
    docker-exim

Note: The syntax is explained here or below.

Direct delivery using MX DNS lookups

If you don't want to use smart hosts, just omit the env var SMARTHOSTS like this:

$ docker run -it --rm \
    --init \
    --name docker-exim \
    -p "25:2525" \
    -v eximspool:/var/spool/exim \
    docker-exim

Modify EHLO/HELO hostname sent

Some servers block messages from servers sending EHLO data which doesn't resolve in DNS. You can override this setting:

$ docker run -it --rm \
    --init \
    --name docker-exim \
    -p "25:2525" \
    -v eximspool:/var/spool/exim \
    -e HELO_DATA="mail.example.com" \
    docker-exim

Note: the $primary_hostname var in exim is still the result of hostname().

About multiple smarthosts

The smarthost list should follow the exim specs about host lists. For example if you want mx1.example.com and mx2.example.com to both act as smart hosts (exim tries the first, then the second etc.), specify the SMARTHOSTS env var as follows:

-e SMARTHOSTS="mx1.example.com : mx2.example.com"

or with ports other than 25:

-e SMARTHOSTS="mx1.example.com::1025 : mx2.example.com::8025"  # yes, double colon (::)

See also: https://www.exim.org/exim-html-current/doc/html/spec_html/ch-the_manualroute_router.html

Security

I wanted a container where exim does not use its root privileges since it doesn't need to deliver locally. Here exim runs as user exim and the harden.sh script should remove a lot of possible attack vectors :)

Size

I tried to keep the Docker image small when building it.

Notes

• The /var/log/exim/main log is redirected to stdout. This way it can be logged by Docker log drivers.
• I don't know why yet but exim doesn't exit when the container wants to stop. Running the container with --init helps.

Thanks to

• @jumanjiman for the gists
• @adegtyarev for the inspiration