It supports have_enqueud_mail matcher sub class of DeliveryJob.

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 SubClassDeliveryJob < ActionMailer::DeliveryJob
+    end
+
+    class SubClassUnifiedMailer < ActionMailer::Base
+      self.delivery_job = SubClassDeliveryJob
+
+      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 subclass delivery_job" do
+        expect {
+          SubClassUnifiedMailer.test_email.deliver_later
+        }.to have_enqueued_mail(SubClassUnifiedMailer, :test_email)
+      end
     end
   end
 end