In e-mail validator, fallback from MX to A record (#7955)

Fix #7831

app/validators/email_mx_validator.rb

@@ -16,6 +16,8 @@ class EmailMxValidator < ActiveModel::Validator
     return true if domain.nil?
 
     records = Resolv::DNS.new.getresources(domain, Resolv::DNS::Resource::IN::MX).to_a.map { |e| e.exchange.to_s }
+    records = Resolv::DNS.new.getresources(domain, Resolv::DNS::Resource::IN::A).to_a.map { |e| e.exchange.to_s } if records.empty?
+
     records.empty? || on_blacklist?(records)
   end