Last modified on Fri 24 Jul 2020



In most cases we don't need a custom design for the emails. We use simple HTML templates and modify them slightly with project's theme colors, and add a logo if needed.

For implementing a fully custom template we use MJML with the mjml-rails gem. Before setting it up on a new project, check with someone from the JS team whether this still is the best tool for the job.

If somebody from the other team is expected to work on emails, please make sure you first set up the Letter opener, prepare the previews for all mailers and instruct your colleague on how to use the mailer previews.

Send emails asynchronously

Emails should be sent asynchronously from a background job:

See the Background jobs chapter on how to configure those.

If you're using Devise, don't forget to configure it for ActiveJob as described here.

Local development

We usually don't need nor want to send real emails to users in development, so we use Letter opener. Instead of sending an email, it will render the email in a new tab.

Mailer previews

Rails has a built-in feature for previewing emails. Using it allows you to see what the email looks like without sending a real email. Please read the setup instructions on the previous link.

We typically add mailer preview classes to the spec/mailers/previews folder.

Here's an example of a preview class:

# Preview all emails at http://localhost:3000/rails/mailers
class UserMailerPreview < ActionMailer::Preview
  def storage_limit_warning_email


  def user '', username: 'mockyMock', space_usage: 450)

The important thing is not to load existing objects from your development database here, because you will have problems when you delete or change the data. Other developers working on the project probably won't have the same data in their database. A better solution would be to just build the necessary objects with all attributes and relationships used in the template.

Production and staging

In production/staging/uat we mainly use Mailgun to send emails from our apps.

Someone from the DevOps team should open a Mailgun account and set up everything for production/staging and store credentials in Vault.

To set up Mailgun in the app, add the official mailgun-ruby gem and set it up per environment as described in the gem's Readme.

Mailgun has webhooks, which are useful if you need to track failed deliveries caused by non-existing/non-verified emails in the database. We've created the mailgun_catcher gem to simplify webhook integration in our apps.

Sending emails on behalf of users

Sometimes we need to send an email on behalf of the app's users, i.e. Contact form submissions. Never set from field to user's email address - use Reply-To.. Mailgun won't deliver emails with from address with a different domain than the one configured in Mailgun settings.

Intercepting emails

Emails in non-production environments should have prefixed subjects, i.e. [Staging] Confirmation Instructions. Creating a custom interceptor is simple, for more info read here.

In some apps you should intercept emails in staging to prevent sending them to real users, or to avoid Mailgun failures if you have fake/non-existing emails in the database. There's the mail_interceptor gem which intercepts and forwards emails to a given address, and also sends the emails only to whitelisted addresses.