Add feature flag for using Received header for incoming emails

This flag - use_received_header_for_incoming_emails - is enabled by
default, but it's there so we can disable it if needed.

config/feature_flags/development/use_received_header_for_incoming_emails.yml

+---
+name: use_received_header_for_incoming_emails
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81489
+rollout_issue_url:
+milestone: '14.9'
+type: development
+group: group::certify
+default_enabled: true

lib/gitlab/email/receiver.rb

@@ -148,6 +148,8 @@ module Gitlab
       end
 
       def find_first_key_from_received_headers
+        return unless ::Feature.enabled?(:use_received_header_for_incoming_emails, default_enabled: :yaml)
+
         recipients_from_received_headers.find do |email|
           key = email_class.key_from_address(email)
           break key if key

spec/lib/gitlab/email/receiver_spec.rb

@@ -38,6 +38,15 @@ RSpec.describe Gitlab::Email::Receiver do
     end
   end
 
+  shared_examples 'failed receive' do
+    it 'adds metric event' do
+      expect(::Gitlab::Metrics::BackgroundTransaction).to receive(:current).and_return(metric_transaction)
+      expect(metric_transaction).to receive(:add_event).with('email_receiver_error', { error: expected_error.name })
+
+      expect { receiver.execute }.to raise_error(expected_error)
+    end
+  end
+
   context 'when the email contains a valid email address in a header' do
     before do
       stub_incoming_email_setting(enabled: true, address: "incoming+%{key}@appmail.example.com")
@@ -75,21 +84,24 @@ RSpec.describe Gitlab::Email::Receiver do
       it_behaves_like 'successful receive'
     end
 
-    context 'when all other headers are missing, fall back to checking Received headers' do
+    context 'when all other headers are missing' do
       let(:email_raw) { fixture_file('emails/missing_delivered_to_header.eml') }
       let(:meta_key) { :received_recipients }
       let(:meta_value) { ['incoming+gitlabhq/gitlabhq+auth_token@appmail.example.com', 'incoming+gitlabhq/gitlabhq@example.com'] }
 
-      it_behaves_like 'successful receive'
-    end
-  end
+      context 'when use_received_header_for_incoming_emails is enabled' do
+        it_behaves_like 'successful receive'
+      end
 
-  shared_examples 'failed receive' do
-    it 'adds metric event' do
-      expect(::Gitlab::Metrics::BackgroundTransaction).to receive(:current).and_return(metric_transaction)
-      expect(metric_transaction).to receive(:add_event).with('email_receiver_error', { error: expected_error.name })
+      context 'when use_received_header_for_incoming_emails is disabled' do
+        let(:expected_error) { Gitlab::Email::UnknownIncomingEmail }
 
-      expect { receiver.execute }.to raise_error(expected_error)
+        before do
+          stub_feature_flags(use_received_header_for_incoming_emails: false)
+        end
+
+        it_behaves_like 'failed receive'
+      end
     end
   end