docker run –name mycontainer –link “postfix:postfix” myimage

Using docker-compose

Or, you could use docker-compose to start your application containers linked against the postfix container with one command. Suppose you have a web application that links against a database and this postfix container. Download and install docker-compose for your platform, and then on your website’s docker project, create a new file called docker-compose.yml, and put the following contents there:

myapp:
build: myapp
ports:
– “80:80”
# If running behind a proxy container, expose the ports instead
# and link the proxy container to this one.
#  expose:
#  – “80”
links:
– mariadb:mariadb
– postfix:postfix
volumes_from:
– data
mariadb:
image: centos/mariadb:latest
expose:
– “3306”
volumes_from:
– data
environment:
MYSQL_ROOT_PASSWORD: changeme
postfix:
image: juanluisbaptiste/postfix:latest
expose:
– “25”
environment:
SMTP_SERVER: smtp.mycompany.com
SMTP_USERNAME: user@mycompany.com
SMTP_PASSWORD: changeme
SERVER_HOSTNAME: helpdesk.mycompany.com

data:
image: centos:latest
volumes:
– /var/lib/mysql
– /var/www/webapp

command: /bin/true

Then, you can launch your webapp, the database and the postfix container with this command:

docker-compose up

All containers will be started in the right order and the webapp will be linked against the mariadb and postfix containers. Also, the webapp and the mariadb database container will share the same data volume container (unrelated to the postfix container but a good practice).

On thing to note, this container doesn’t enable client SMTP authentication, the idea is to expose the port 25 to containers, and then link the containers that need a SMTP service against it so the relay isn’t publicly exposed.

A note about using gmail as a relay

Since last year, Gmail by default does not allow email clients that don’t use OAUTH 2
for authentication (like Thunderbird or Outlook). First you need to enable access to “Less secure apps” on your google settings.

Also take into account that email From: header will contain the email address of the account being used to authenticate against the Gmail SMTP server (SMTP_USERNAME), the one on the email will be ignored by Gmail.