Config to use local nameservers for DNS validation

app/models/domain.rb

@@ -135,7 +135,7 @@ class Domain < ApplicationRecord
   end
 
   def resolver
-    @resolver ||= Resolv::DNS.new(:nameserver => nameservers)
+    @resolver ||= Postal.config.general.use_local_ns_for_domains? ? Resolv::DNS.new : Resolv::DNS.new(:nameserver => nameservers)
   end
 
   private

config/postal.defaults.yml

@@ -12,6 +12,7 @@ general:
   use_ip_pools: false
   exception_url:
   maximum_delivery_attempts: 18
+  use_local_ns_for_domains: false
 
 web_server:
   bind_address: 127.0.0.1