Don't mutate the Base settings by merge!ing the given value

actionmailer/lib/action_mailer/delivery_methods.rb

@@ -64,7 +64,7 @@ module ActionMailer
           raise "Delivery method cannot be nil"
         when Symbol
           if klass = delivery_methods[method]
-            mail.delivery_method(klass,(send(:"#{method}_settings") || {}).merge!(options || {}))
+            mail.delivery_method(klass, (send(:"#{method}_settings") || {}).merge(options || {}))
           else
             raise "Invalid delivery method #{method.inspect}"
           end

actionmailer/test/delivery_methods_test.rb

@@ -152,6 +152,9 @@ class MailDeliveryTest < ActiveSupport::TestCase
     assert_equal "overridden", delivery_method_instance.settings[:user_name]
     assert_equal "somethingobtuse", delivery_method_instance.settings[:password]
     assert_equal delivery_method_instance.settings.merge(overridden_options), delivery_method_instance.settings
+
+    # make sure that overriding delivery method options per mail instance doesn't affect the Base setting
+    assert_equal settings, ActionMailer::Base.smtp_settings
   end
 
   test "non registered delivery methods raises errors" do