Deprecate custom Action Mailer delivery job:

- Action Mailer delivery job should modify their `perform` method
  signature in order to receive the new payload that Action Mailer
  sends.

  Before:

  ```ruby
    def perform(mailer, mail_method, delivery_method, *args)
    end
  ```

  After:

  ```ruby
    def perform(mailer, mail_method, delivery_method, args:)
    end
  ```

  This new behaviour was introduced couple years ago in a attempt to
  get rid of the necessity to have a different job for paramterized
  mailers. A deprecation was introduced for custom jobs inheriting
  from `ActionMailer::DeliveryJob` but for jobs that didn't it went
  unnoticed.
  The deprecated behaviour was supposed to be removed in Rails 6.1
  but we couldn't and it got reverted https://github.com/rails/rails/pull/39257

actionmailer/lib/action_mailer/message_delivery.rb

@@ -142,14 +142,37 @@ def enqueue_delivery(delivery_method, options = {})
         else
           job = @mailer_class.delivery_job
 
-          if job <= MailDeliveryJob
+          if use_new_args?(job)
             job.set(options).perform_later(
               @mailer_class.name, @action.to_s, delivery_method.to_s, args: @args)
+          elsif job <= DeliveryJob
+            job.set(options).perform_later(
+              @mailer_class.name, @action.to_s, delivery_method.to_s, *@args)
           else
+            ActiveSupport::Deprecation.warn(<<~EOM)
+              In Rails 6.2, Action Mailer will pass the mail arguments inside the `:args` keyword argument.
+              The `perform` method of the #{job} needs to change and forward the mail arguments
+              from the `args` keyword argument.
+
+              The `perform` method should now look like:
+
+              `def perform(mailer, mail_method, delivery, args:)`
+            EOM
+
             job.set(options).perform_later(
               @mailer_class.name, @action.to_s, delivery_method.to_s, *@args)
           end
         end
       end
+
+      def use_new_args?(job)
+        parameters = job.public_instance_method(:perform).parameters
+
+        parameters.find do |key, name|
+          return true if key == :keyreq && name == :args
+
+          key == :keyrest
+        end
+      end
   end
 end

actionmailer/test/legacy_delivery_job_test.rb

@@ -11,6 +11,21 @@ class LegacyDeliveryJobTest < ActiveSupport::TestCase
   class LegacyDeliveryJob < ActionMailer::DeliveryJob
   end
 
+  class LegacyArgumentDeliveryJob < ActiveJob::Base
+    def perform(mailer, mail_method, delivery_method, *args)
+    end
+  end
+
+  class NewArgumentDeliveryJob < ActiveJob::Base
+    def perform(mailer, mail_method, delivery_method, args:)
+    end
+  end
+
+  class KeyRestArgumentJob < ActiveJob::Base
+    def perform(mailer, mail_method, delivery_method, **kwargs)
+    end
+  end
+
   setup do
     @previous_logger = ActiveJob::Base.logger
     ActiveJob::Base.logger = Logger.new(nil)
@@ -68,6 +83,36 @@ class LegacyDeliveryJob < ActionMailer::DeliveryJob
     end
   end
 
+  test "triggers a deprecation warning when a delivery job use legacy arguments" do
+    with_delivery_job(LegacyArgumentDeliveryJob) do
+      assert_deprecated("Action Mailer will pass the mail arguments inside the `:args` keyword argument") do
+        perform_enqueued_jobs do
+          DelayedMailer.test_message(1, 2, 3).deliver_later
+        end
+      end
+    end
+  end
+
+  test "does not trigger a deprecation warning when a delivery job use a required `args` kwargs" do
+    with_delivery_job(NewArgumentDeliveryJob) do
+      assert_not_deprecated do
+        perform_enqueued_jobs do
+          DelayedMailer.test_message(1, 2, 3).deliver_later
+        end
+      end
+    end
+  end
+
+  test "does not trigger a deprecation warning when a delivery job use a keyrest argument" do
+    with_delivery_job(KeyRestArgumentJob) do
+      assert_not_deprecated do
+        perform_enqueued_jobs do
+          DelayedMailer.test_message(1, 2, 3).deliver_later
+        end
+      end
+    end
+  end
+
   private
     def with_delivery_job(job)
       old_params_delivery_job = ParamsMailer.delivery_job