Customize ActionMailer preview path

config/environments/development.rb

@@ -39,6 +39,7 @@ Rails.application.configure do
   config.action_mailer.delivery_method = :letter_opener_web
   # Don't make a mess when bootstrapping a development environment
   config.action_mailer.perform_deliveries = (ENV['BOOTSTRAP'] != '1')
+  config.action_mailer.preview_path = 'spec/mailers/previews'
 
   config.eager_load = false
 end

spec/mailers/previews/devise_mailer_preview.rb

+class DeviseMailerPreview < ActionMailer::Preview
+  def confirmation_instructions_for_signup
+    user = User.new(email: 'signup@example.com')
+    DeviseMailer.confirmation_instructions(user, 'faketoken', {})
+  end
+
+  def confirmation_instructions_for_new_email
+    user = User.last
+    DeviseMailer.confirmation_instructions(user, 'faketoken', {})
+  end
+end