How to stop staging app from emailing real users

Having production data in a staging environment makes it easier to catch bugs.

But when testing a feature, there's always that tiny moment of hesitation when you hit the "Send Email" button.

Will this actually go out to a real user?

I'm sure most of you have been there.

For local development, we usually install the excellent letter_opener gem. But what about staging?

Safely sending emails in staging

Rails 7.1+ introduced a new before_deliver callback for Action Mailers, which is perfect for overriding recipients before emails are actually sent.

Here's how to use it:

class ApplicationMailer < ActionMailer::Base
  before_deliver :override_recipients

  private

  def override_recipients
    return unless Rails.env.staging?

    sandbox_emails = ENV.fetch("SANDBOX_EMAILS")

    message.to = sandbox_emails
    message.cc = sandbox_emails
    message.bcc = sandbox_emails
  end
end

Just make sure you have the SANDBOX_EMAILS environment variable set. Or pull it from Rails credentials if you prefer.

If you're on an older version of Rails (< 7.1), you can use interceptor to accomplish the same thing:

# app/mailers/sandbox_email_interceptor.rb

class SandboxEmailInterceptor
  def self.delivering_email(message)
    sandbox_emails = ENV.fetch("SANDBOX_EMAILS")

    message.to = sandbox_emails
    message.cc = sandbox_emails
    message.bcc = sandbox_emails
  end
end

The interceptor needs to be registered using the interceptors option of action_mailer.

# config/initializers/mail_interceptors.rb

Rails.application.configure do
  if Rails.env.staging?
    config.action_mailer.interceptors = %w[SandboxEmailInterceptor]
  end
end

That is it. You can now click "Send Email" without the panic.

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