diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ac2b0bd7b4..db17fd9242 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -16,23 +16,40 @@ jobs:
       fail-fast: false
       matrix:
        include:
-         # Rails 6.1 builds >= 2.5
-         - ruby: ruby-3.0.0-preview1
+         # Rails Master builds >= 2.5
+         - ruby: ruby-3.0.0-preview2
            allow_failure: true
            env:
              RAILS_VERSION: 'master'
          - ruby: 2.7.1
+           allow_failure: true
            env:
              RAILS_VERSION: 'master'
          - ruby: 2.6.6
+           allow_failure: true
            env:
              RAILS_VERSION: 'master'
          - ruby: 2.5.8
+           allow_failure: true
            env:
              RAILS_VERSION: 'master'
 
+         # Rails 6.1.0 builds >= 2.5
+         - ruby: ruby-3.0.0-preview2
+           env:
+             RAILS_VERSION: '~> 6.1.0'
+         - ruby: 2.7.1
+           env:
+             RAILS_VERSION: '~> 6.1.0'
+         - ruby: 2.6.6
+           env:
+             RAILS_VERSION: '~> 6.1.0'
+         - ruby: 2.5.8
+           env:
+             RAILS_VERSION: '~> 6.1.0'
+
          # Rails 6.0 builds >= 2.5.0
-         - ruby: 3.0.0-preview1
+         - ruby: 3.0.0-preview2
            env:
              RAILS_VERSION: '~> 6.0.0'
          - ruby: 2.7
diff --git a/example_app_generator/generate_action_mailer_specs.rb b/example_app_generator/generate_action_mailer_specs.rb
index 13a1f0a4c0..06833fc1a5 100644
--- a/example_app_generator/generate_action_mailer_specs.rb
+++ b/example_app_generator/generate_action_mailer_specs.rb
@@ -14,15 +14,20 @@
       end
     end
   CODE
-  gsub_file 'config/initializers/action_mailer.rb',
-            /ExampleApp/,
-            Rails.application.class.parent.to_s
+
+  rails_parent =
+    if Rails.version.to_f >= 6.0
+      Rails.application.class.module_parent.to_s
+    else
+      Rails.application.class.parent.to_s
+    end
+
+  gsub_file 'config/initializers/action_mailer.rb', /ExampleApp/, rails_parent
 
   copy_file 'spec/support/default_preview_path'
   chmod 'spec/support/default_preview_path', 0755
-  gsub_file 'spec/support/default_preview_path',
-            /ExampleApp/,
-            Rails.application.class.parent.to_s
+  gsub_file 'spec/support/default_preview_path', /ExampleApp/, rails_parent
+
   if skip_active_record?
     comment_lines 'spec/support/default_preview_path', /active_record/
     comment_lines 'spec/support/default_preview_path', /active_storage/
diff --git a/features/controller_specs/anonymous_controller.feature b/features/controller_specs/anonymous_controller.feature
index 7892b4dfac..9baf73fb8d 100644
--- a/features/controller_specs/anonymous_controller.feature
+++ b/features/controller_specs/anonymous_controller.feature
@@ -101,6 +101,8 @@ Feature: anonymous controller
     When I run `rspec spec`
     Then the examples should all pass
 
+  # Deprecated support removed in https://github.com/rails/rails/commit/d52d7739468153bd6cb7c629f60bd5cd7ebea3eb
+  @rails_pre_6
   Scenario: Specify error handling in `ApplicationController` with render :file
     Given a file named "spec/controllers/application_controller_spec.rb" with:
       """ruby
diff --git a/lib/rspec/rails/matchers/action_mailbox.rb b/lib/rspec/rails/matchers/action_mailbox.rb
index c5392cb225..de50c69143 100644
--- a/lib/rspec/rails/matchers/action_mailbox.rb
+++ b/lib/rspec/rails/matchers/action_mailbox.rb
@@ -22,11 +22,20 @@ def initialize(message)
             @inbound_email = create_inbound_email(message)
           end
 
-          def matches?(mailbox)
-            @mailbox  = mailbox
-            @receiver = ApplicationMailbox.router.send(:match_to_mailbox, inbound_email)
+          if defined?(::ApplicationMailbox) && ::ApplicationMailbox.router.respond_to?(:mailbox_for)
+            def matches?(mailbox)
+              @mailbox  = mailbox
+              @receiver = ApplicationMailbox.router.mailbox_for(inbound_email)
 
-            @receiver == @mailbox
+              @receiver == @mailbox
+            end
+          else
+            def matches?(mailbox)
+              @mailbox  = mailbox
+              @receiver = ApplicationMailbox.router.send(:match_to_mailbox, inbound_email)
+
+              @receiver == @mailbox
+            end
           end
 
           def failure_message
@@ -41,7 +50,7 @@ def failure_message_when_negated
             "expected #{describe_inbound_email} not to route to #{mailbox}"
           end
 
-        private
+          private
 
           attr_reader :inbound_email, :mailbox, :receiver