Clear test mailbox from ActionMailer::Base between each example

We notice in #2290 that
ActionMailer::Base.deliveries mailbox is not cleaned between example.

Fix: #2290

lib/rspec/rails/configuration.rb

@@ -137,6 +137,7 @@ def filter_rails_from_backtrace!
 
       if RSpec::Rails::FeatureCheck.has_action_mailer?
         config.include RSpec::Rails::MailerExampleGroup, type: :mailer
+        config.after { ActionMailer::Base.deliveries.clear }
       end
 
       if RSpec::Rails::FeatureCheck.has_active_job?

spec/rspec/rails/configuration_spec.rb

@@ -256,6 +256,31 @@ def self.application; end
       expect(group.mailer_class).to be(a_mailer_class)
       expect(group.new).to be_a(RSpec::Rails::MailerExampleGroup)
     end
+
+    describe 'cleans test mailbox between each example in all rspec-rails example' do
+      class BaseMailer < ActionMailer::Base
+        default from: '[email protected]'
+
+        def welcome(to:)
+          mail(to: to, subject: 'subject', body: render(inline: "Hello", layout: false))
+        end
+      end
+      before do
+        ActionMailer::Base.delivery_method = :test
+      end
+
+      it 'send to email@' do
+        BaseMailer.welcome(to: '[email protected]').deliver_now
+
+        expect(ActionMailer::Base.deliveries.map(&:to).flatten.sort).to eq(['[email protected]'])
+      end
+
+      it 'send to email_2@' do
+        BaseMailer.welcome(to: '[email protected]').deliver_now
+
+        expect(ActionMailer::Base.deliveries.map(&:to).flatten.sort).to eq(['[email protected]'])
+      end
+    end
   end
 
   it "has a default #file_fixture_path of 'spec/fixtures/files'" do