Only configure smtp_settings if provided on configuration

actionmailer/lib/action_mailer/railtie.rb

@@ -23,7 +23,6 @@ module ActionMailer
       options.stylesheets_dir ||= paths["public/stylesheets"].first
       options.show_previews = Rails.env.development? if options.show_previews.nil?
       options.cache_store ||= Rails.cache
-      options.smtp_settings ||= {}
 
       if options.show_previews
         options.preview_path ||= defined?(Rails.root) ? "#{Rails.root}/test/mailers/previews" : nil
@@ -46,9 +45,15 @@ module ActionMailer
           self.delivery_job = delivery_job.constantize
         end
 
-        if smtp_timeout = options.delete(:smtp_timeout)
-          options.smtp_settings[:open_timeout] ||= smtp_timeout
-          options.smtp_settings[:read_timeout] ||= smtp_timeout
+        if options.smtp_settings
+          self.smtp_settings = options.smtp_settings
+        end
+
+        smtp_timeout = options.delete(:smtp_timeout)
+
+        if self.smtp_settings && smtp_timeout
+          self.smtp_settings[:open_timeout] ||= smtp_timeout
+          self.smtp_settings[:read_timeout] ||= smtp_timeout
         end
 
         options.each { |k, v| send("#{k}=", v) }

railties/test/application/configuration_test.rb

@@ -2799,7 +2799,36 @@ module ApplicationTests
       smtp_settings = { domain: "example.com" }
 
       assert_equal smtp_settings, ActionMailer::Base.smtp_settings
+    end
+
+    test "Rails.application.config.action_mailer.smtp_settings = nil fallback to ActionMailer::Base.smtp_settings" do
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config <<-RUBY
+        ActionMailer::Base.smtp_settings = { domain: "example.com" }
+        config.load_defaults "7.0"
+      RUBY
+
+      app "development"
+
+      smtp_settings = { domain: "example.com", open_timeout: 5, read_timeout: 5 }
+
       assert_equal smtp_settings, ActionMailer::Base.smtp_settings
+      assert_nil Rails.configuration.action_mailer.smtp_settings
+    end
+
+    test "Rails.application.config.action_mailer.smtp_settings = nil and ActionMailer::Base.smtp_settings = nil do not configure smtp_timeout" do
+      ActionMailer::Base.smtp_settings = nil
+
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config <<-RUBY
+        config.action_mailer.smtp_settings = nil
+        config.load_defaults "7.0"
+      RUBY
+
+      app "development"
+
+      assert_nil Rails.configuration.action_mailer.smtp_settings
+      assert_nil ActionMailer::Base.smtp_settings
     end
 
     test "ActiveSupport.utc_to_local_returns_utc_offset_times is true in 6.1 defaults" do