fix(smtp_server): refactor mx lookups to randomly order mx records with the same priority

closes #1408
closes #1405

config/initializers/inflections.rb

@@ -19,5 +19,6 @@ ActiveSupport::Inflector.inflections(:en) do |inflect|
   inflect.acronym 'UUID'
   inflect.acronym 'HTTP'
   inflect.acronym 'DB'
+  inflect.acronym 'MX'
   inflect.acronym 'DKIM'
 end

lib/postal.rb

@@ -19,12 +19,13 @@ module Postal
     autoload :MessageInspection
     autoload :MessageParser
     autoload :MessageRequeuer
+    autoload :MXLookup
     autoload :QueryString
     autoload :RabbitMQ
     autoload :ReplySeparator
     autoload :RspecHelpers
-    autoload :SendResult
     autoload :Sender
+    autoload :SendResult
     autoload :SMTPSender
     autoload :SMTPServer
     autoload :SpamCheck

lib/postal/mx_lookup.rb

@@ -0,0 +1,34 @@
+module Postal
+  class MXLookup
+
+    class << self
+
+      def lookup(domain)
+        records = resolve(domain)
+        records = sort(records)
+        records.map { |m| m[1] }
+      end
+
+      private
+
+      def sort(records)
+        records.sort do |a, b|
+          if a[0] == b[0]
+            [-1, 1].sample
+          else
+            a[0] <=> b[0]
+          end
+        end
+      end
+
+      def resolve(domain)
+        Resolv::DNS.open do |dns|
+          dns.timeouts = [10,5]
+          dns.getresources(domain, Resolv::DNS::Resource::IN::MX).map { |m| [m.preference.to_i, m.exchange.to_s] }
+        end
+      end
+
+    end
+
+  end
+end

lib/postal/smtp_sender.rb

@@ -200,13 +200,9 @@ module Postal
 
     def servers
       @options[:servers] || self.class.relay_hosts || @servers ||= begin
-        mx_servers = []
-        Resolv::DNS.open do |dns|
-          dns.timeouts = [10,5]
-          mx_servers = dns.getresources(@domain, Resolv::DNS::Resource::IN::MX).map { |m| [m.preference.to_i, m.exchange.to_s] }.sort.map{ |m| m[1] }
-          if mx_servers.empty?
-            mx_servers = [@domain] # This will be resolved to an A or AAAA record later
-          end
+        mx_servers = MXLookup.lookup(@domain)
+        if mx_servers.empty?
+          mx_servers = [@domain] # This will be resolved to an A or AAAA record later
         end
         mx_servers
       end