4 files changed, 26 insertions, 11 deletions
diff --git a/actionmailer/lib/rails/generators/mailer/mailer_generator.rb b/actionmailer/lib/rails/generators/mailer/mailer_generator.rb
index 3ec7d3d896..c37a74c762 100644
--- a/actionmailer/lib/rails/generators/mailer/mailer_generator.rb
+++ b/actionmailer/lib/rails/generators/mailer/mailer_generator.rb
@@ -1,24 +1,37 @@
+# frozen_string_literal: true
+
 module Rails
   module Generators
     class MailerGenerator < NamedBase
-      source_root File.expand_path("../templates", __FILE__)
+      source_root File.expand_path("templates", __dir__)
 
       argument :actions, type: :array, default: [], banner: "method method"
 
       check_class_collision suffix: "Mailer"
 
       def create_mailer_file
-        template "mailer.rb", File.join('app/mailers', class_path, "#{file_name}_mailer.rb")
-        if self.behavior == :invoke
-          template "application_mailer.rb", 'app/mailers/application_mailer.rb'
+        template "mailer.rb", File.join("app/mailers", class_path, "#{file_name}_mailer.rb")
+
+        in_root do
+          if behavior == :invoke && !File.exist?(application_mailer_file_name)
+            template "application_mailer.rb", application_mailer_file_name
+          end
         end
       end
 
       hook_for :template_engine, :test_framework
 
-      protected
-        def file_name
-          @_file_name ||= super.gsub(/\_mailer/i, '')
+      private
+        def file_name # :doc:
+          @_file_name ||= super.sub(/_mailer\z/i, "")
+        end
+
+        def application_mailer_file_name
+          @_application_mailer_file_name ||= if mountable_engine?
+            "app/mailers/#{namespaced_path}/application_mailer.rb"
+          else
+            "app/mailers/application_mailer.rb"
+          end
         end
     end
   end
diff --git a/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb b/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb
deleted file mode 100644
index d25d8892dd..0000000000
--- a/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb
+++ /dev/null
@@ -1,4 +0,0 @@
-class ApplicationMailer < ActionMailer::Base
-  default from: "from@example.com"
-  layout 'mailer'
-end
diff --git a/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb.tt b/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb.tt
new file mode 100644
index 0000000000..00fb9bd48f
--- /dev/null
+++ b/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb.tt
@@ -0,0 +1,6 @@
+<% module_namespacing do -%>
+class ApplicationMailer < ActionMailer::Base
+  default from: 'from@example.com'
+  layout 'mailer'
+end
+<% end %>
diff --git a/actionmailer/lib/rails/generators/mailer/templates/mailer.rb b/actionmailer/lib/rails/generators/mailer/templates/mailer.rb.tt
index 348d314758..348d314758 100644
--- a/actionmailer/lib/rails/generators/mailer/templates/mailer.rb
+++ b/actionmailer/lib/rails/generators/mailer/templates/mailer.rb.tt