Exchange 2013, KEMP Load Balancer

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?
totallypatrickAsked:
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totallypatrickConnect With a Mentor Author Commented:
HI Sembre,
 
It's using the DNS server from my AD. I found the issue. Under the Send Connectors, I forgot to tick the Use external DNS lookup Settings on servers with transport roles.
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Simon Butler (Sembee)Connect With a Mentor ConsultantCommented:
If you remove the external DNS records from within Exchange so it is using the domain controllers for DNS, what happens then? If you bypass the load balancers, what happens?

Simon.
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totallypatrickAuthor Commented:
Found the solution to my problem
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