Add DomainResource class to wrap MX lookup/normalize (#32864)

app/lib/domain_resource.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class DomainResource
+  attr_reader :domain
+
+  RESOLVE_TIMEOUT = 5
+
+  def initialize(domain)
+    @domain = domain
+  end
+
+  def mx
+    Resolv::DNS.open do |dns|
+      dns.timeouts = RESOLVE_TIMEOUT
+      dns
+        .getresources(domain, Resolv::DNS::Resource::IN::MX)
+        .to_a
+        .map { |mx| mx.exchange.to_s }
+        .compact_blank
+    end
+  end
+end