Maybe only for Rails 3?

Author: bquorning

example_app_generator/generate_action_mailer_specs.rb

@@ -24,6 +24,11 @@ def comment_lines(path, flag, *args)
         end
       end
     end
+
+    if defined?(ActionMailer) && Rails::VERSION::MAJOR < 4
+      # This will force the loading of ActionMailer settings
+      ActionMailer::Base.smtp_settings
+    end
   CODE
   gsub_file 'config/initializers/action_mailer.rb',
             /ExampleApp/,

example_app_generator/spec/support/default_preview_path

@@ -32,6 +32,7 @@ require_file_stub 'config/environment' do
     module ExampleApp
       class Application < Rails::Application
         config.eager_load = false
+        config.eager_load_paths.clear if Rails::VERSION::MAJOR < 4
 
         # Don't care if the mailer can't send.
         config.action_mailer.raise_delivery_errors = false unless ENV['NO_ACTION_MAILER']