It supports have_enqueud_mail matcher sub class of DeliveryJob.

Changelog.md

@@ -5,6 +5,7 @@ Bug Fixes:
 
 * Remove warning when calling `driven_by` in system specs. (Aubin Lorieux, #2302)
 * Fix comparison of times for `#at` in job matchers. (Jon Rowe, Markus Doits, #2304)
+* Fix when using a mailer with `delivery_job` set to a sub class of `ActionMailer::DeliveryJob` (Atsushi Yoshida #2305)
 
 ### 4.0.0 / 2020-03-24
 [Full Changelog](https://github.com/rspec/rspec-rails/compare/v3.9.1...v4.0.0)

lib/rspec/rails/matchers/have_enqueued_mail.rb

@@ -131,15 +131,15 @@ def mail_job_message(job)
         end
 
         def legacy_mail?(job)
-          job[:job] == ActionMailer::DeliveryJob
+          job[:job] <= ActionMailer::DeliveryJob
         end
 
         def parameterized_mail?(job)
-          RSpec::Rails::FeatureCheck.has_action_mailer_parameterized? && job[:job] == ActionMailer::Parameterized::DeliveryJob
+          RSpec::Rails::FeatureCheck.has_action_mailer_parameterized? && job[:job] <= ActionMailer::Parameterized::DeliveryJob
         end
 
         def unified_mail?(job)
-          RSpec::Rails::FeatureCheck.has_action_mailer_unified_delivery? && job[:job] == ActionMailer::MailDeliveryJob
+          RSpec::Rails::FeatureCheck.has_action_mailer_unified_delivery? && job[:job] <= ActionMailer::MailDeliveryJob
         end
       end
       # @api public

spec/rspec/rails/matchers/have_enqueued_mail_spec.rb

@@ -21,6 +21,15 @@ class UnifiedMailer < ActionMailer::Base
       def test_email; end
       def email_with_args(arg1, arg2); end
     end
+
+    class DeliveryJobSubClass < ActionMailer::DeliveryJob
+    end
+
+    class UnifiedMailerWithDeliveryJobSubClass < ActionMailer::Base
+      self.delivery_job = DeliveryJobSubClass
+
+      def test_email; end
+    end
   end
 end
 
@@ -397,6 +406,12 @@ def self.name; "NonMailerJob"; end
           a_hash_including(params: {'foo' => 'bar'}, args: [1, 2])
         )
       end
+
+      it "passes when using a mailer with `delivery_job` set to a sub class of `ActionMailer::DeliveryJob`" do
+        expect {
+          UnifiedMailerWithDeliveryJobSubClass.test_email.deliver_later
+        }.to have_enqueued_mail(UnifiedMailerWithDeliveryJobSubClass, :test_email)
+      end
     end
   end
 end