Ditch module syntax and use namespacing instead. Here we will use an existing mailer as an example, but you can namespace anything in our project: controller, models, helpers, you name it!

1. Place the mailer in the subdirectory ex mailers/users/

2. Add the namespace to the mailer class itself

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   class Users::AvailabilityMailingListNewSubscriptionMailer < ApplicationMailer

3. Wherever the mailer is called in the application, preface it with the namespace

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   def perform(zipcode, email)
     @zipcode = zipcode
     @email = email
     Users::AvailabilityMailingListNewSubscriptionMailer.send_subscribed_notice(
       @zipcode,
       @email
     )
       .deliver_later
   end

4. namespace the appropriate spec folder to match the subfolder in the application spec/mailers/users/

5. namespace the spec file

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   require "rails_helper"
   
   RSpec.describe Users::AvailabilityMailingListNewSubscriptionMailer, type: :mailer do

6. namespace the mailer preview as well spec/mailers/previews/users/

7. namespace the mailer preview file

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   class Users::AvailabilityMailingListNewSubscriptionPreview < ActionMailer::Preview