How to send mail from Ubuntu server using dockerized mail server

This is similar to having a Jenkins container which must send emails, as described here:

For containerized Jenkins system, mail server can also be configured in same Manage Jenkins page, E-mail Notification section.
The only difference is the IP/hostname provided to SMTP server option. Instead of providing the known SMTP server’s IP and host, one should use the IP of docker0.

For corporate network, you may have to use an SMTP relay server instead. For those cases, you can configure SMTP communication by setting up Postfix.

After installing, update /etc/postfix/main.cf with correct relay information: myhostname, myorigin, mydestination, relayhost, alias_maps, alias_database.

But:

There are two changes need to be made on Postfix to expose it to Docker containers on one host.

1. Exposing Postfix to the docker network, that is, Postfix must be configured to bind to localhost as well as the docker network.
2. Accepting all incoming connections which come from any Docker containers.

Docker bridge (docker0) acts a a bridge between your ethernet port and docker containers so that data can go back and forth.
We achieve the first requirement by adding the IP of docker0 to inet_iterfaces.

For the second requirement, the whole docker network as well as localhost should be added to mynetworks.

For this problem, the easiest solution I have found is nodemailer, as my application that needs to send the emails is a node.js application, and I connect to it as you would from an email client.