Dcycle Blog

Debug outgoing emails with Mailhog, a dummy mailserver with a GUI

March 14, 2019

Often, during local Drupal development (or if we’re really unlucky, in production), we get the dreaded message, “Unable to send e-mail. Contact the site administrator if the problem persists.”

This can make it hard to debug anything email-related during local development.

Enter Mailhog

Mailhog is a dummy SMTP server with a browser GUI, which means you view all outgoing messages with a Gmail-type interface.

It is a major pain to install, but we can automate the entire process with the magic of Docker.

Let’s see how it works, and discuss after. Follow along by installing Docker Desktopno other dependencies are required – and installing a Drupal 8 starterkit:

git clone https://github.com/dcycle/starterkit-drupal8site.git
cd starterkit-drupal8site

This will install the following Docker containers: a MySQL server with a starter database, a configured Drupal site, and Mailhog. You wil see something like this at the end of the output:

If all went well you can now access your site at:

=> Drupal:
=> Dummy email client:

You might be seeing different port numbers instead of 32791 and 32790, so use your own instead of the example ports.

Now, the magic

  • Visit
  • Enter “admin” and submit
  • Now visit

(In my example, DRUPAL_PORT is 32791 and MAILHOG_PORT is 32790. In your case it will probably be different.)

As you can see, all emails produced by Drupal are now visible on a cool GUI!

So how does it work?

A dedicated “Mailhog” docker container, using on the Mailhog Docker image is defined in our docker-compose.yml file. It exposes port 8025 for public GUI access, which is mapped to a random unused port on the host computer (in the above example, 32790). Port 1025 is the SMTP mailhog port as you can see in the Mailhog Dockerfile. We are not mapping port 1025 to a random port on the host computer because it’s only needed in the Drupal container, not the host machine.

In the same docker-compose.yml, the “drupal” container (service) defines a link to the “mail” service; this means that when you are inside the Drupal container, you can access Mailhog SMPT server “mail” at port 1025.

In the Starterkit’s Dockerfile, we download the SMTP modules, and in our configuration, we install SMTP (0, in this case, is the module’s weight, it doesn’t mean “disabled”!).

Next, configuration: because this is for local development, we are leaving SMTP off in the exported configuration; in production we don’t want SMTP to link to Mailhog. Then, in our overridden settings, we enable SMTP and set the server to “mail” and the port to 1025.

Now, you can debug sent emails in a very realistic way!

You can remove the starterkit environment by running:

docker-compose down -v