[Docker] How to Create a Simple Email Forwarder in 2024

Few registrars or hosting companies provide free email forwarding services nowadays. So you have to either pay for mail forwarding for your domain, or you could create your own email forwarder.

Let’s create a email forwarder in a Docker container. It is probably the simplest solution. You can use it, for example on a VPS server.

We’ll be using the Simple Mail Forwarder zixia/simple-mail-forwarder to create a mail forwarder in a Docker container. It is pretty well documented. Please see the documentation on Docket Hub and GitHub). We’ll just give an example of configuring a mail forwarder with Docker Compose here.

Suppose, you own the domains domain1.com and domain2.com. Both sites are hosted (as virtual hosts) on the same VPS server. You want to forward the domain mail for both domains like this:

• box1@domain1.com => mybox@gmail.com
• box2@domain1.com => mybox@gmail.com
• box1@domain2.com => mybox@gmail.com

Before running the example below, please replace all these emails with your real email addresses.

To add the mail forwarder:

1. Add MX Records

Add MX records for all domains where you plan to receive or forward mail to domain email addresses.

Adding an MX record is very easy. We’ll give an example for GoDaddy DNS servers:

1. Sign in to your GoDaddy account
2. Scroll down to the Domains table
3. Click DNS next to the domain where you wish to add an MX record
4. In the block “DNS Records” click the button “Add” on the top right
5. Enter an MX record:

• Type: MX
• Name: @
  This is the same as the main domain name. E.g. if these are DNS records for domain1.com, @ would be the same as domain1.com.
• Priority: 10
  You can put any (0 greater) integer number here.
  SMTP servers will try to deliver mail first to mail servers defined by MX records with lower priority.
• Value: domain1.com
  It must contain a domain name (not an IP address) of the host, responsible for receiving mail for the domain, defined in the Name section of this MX record.
  Sometimes, you need to set up a separate server for receiving mail for your domain. In this case, put that server domain name into the Value, for example, mail.domain1.com.
• TTL: 1 Hour
  Time To Live. This is how much time other DNS servers should wait before updating this record on their side. You could set any time you wish here.

Also, please see this article on adding an MX record at GoDaddy.

Adding an MX record is usually similar and very straightforward on other registrars.

2. Configure the Docker Container

Create a file compose.yaml (in this example, it was created in the folder /opt/projects/mail_forwarder/ on an Ubuntu 20.04 server):

services:

  smf:
    container_name: mail-forwarder
    image: zixia/simple-mail-forwarder
    restart: always

    ports:
      - "25:25"

    environment:
      - SMF_CONFIG=box1@domain1.com:mybox@gmail.com;box2@domain1.com:mybox@gmail.com;box1@domain2.com:mybox@gmail.com

    volumes:
      - /opt/projects/mail_forwarder/volumes/logs:/var/log/postfix

Now, to start the email forwarder, run in the terminal:

docker compose up -d

If you need to run the container but to rebuild the image first:

docker compose up -d --build

To stop the container (and remove the container and everything created by “up”):

docker compose down

If you ever need to enter the Docker container terminal, use the command:

docker exec -it mail-forwarder bash

, where mail-forwarder is the name of the container.

You can see the list of currently running containers with:

docker ps

Please see the Docker documentation for more Docker commands.

Conclusion

Setting up an email forwarder in a Docker container should be a quick and simple solution. It has some advantages:

• Easy to set up
• Easy to remove if necessary
  You just stop the Docker container. You can also remove the image (optional).
• Your container should be isolated from the host machine pretty reliably.
  Docker uses iptables for network isolation. It is pretty bulletproof.

Still, this was a very basic example of the Simple Mail Forwarder configuration. In addition, you may want to use the TLS (SSL) certificate, set up DKIM keys, etc. For these please see the Simple Mail Forwarder documentation on the Docker Hub and GitHub.

If you do not need an email forwarder, but just need to be able to send e-mails from web forms of a WordPress site in a Docker container, please see this article.

Also, if you do not want to experiment in the production environment, you could try things in the local development environment first. In this case, please see this article on how to set up the local development environment on a VirtualBox virtual machine.

Thank you very much for reading to the end.