Dear experts, I'm running 2 X Exchange 2013 (all in one roles) in DAG mode. Both servers are behind a KEMP load balancer. The gateway of the Exchange servers are pointing to the KEMP LB. I have a problem where mail to a certain domain experience this error "451 4.4.0 dns query failed". Under Exchange Settings in ECP and in DNS lookup under each server, I have used external DNS servers (including Gmail DNS server). I can run nslookup in each Exchange server to that domain with no problem. It can resolve the MX records. Also mail to external domains takes between 2 to 3 minutes to receive. The server is not heavily utilized. May I know what could be the problem?