Add explicit DNS timeout

This makes DNS timeouts occur much more quickly and avoid clients with nonfunctional reverse DNS timing out while waiting for a response to their EHLO requests.

lib/postal/smtp_server/client.rb

@@ -105,7 +105,10 @@ module Postal
       private
 
       def resolve_hostname
-        @hostname = Resolv.new.getname(@ip_address) rescue @ip_address
+        Resolv::DNS.open do |dns|
+          dns.timeouts = [10,5]
+          @hostname = dns.getname(@ip_address) rescue @ip_address
+        end
       end
 
       def proxy(data)