Recently we have inbound mail stuck at Edge server with error 451 4.4.0 DNS Query Failed..due to primary DNS was shutdown. After we switch on primary DNS , the mail flow to internal became normal.
As Microsoft mentioned it was due to this problem commonly occurs as a result of a mistake in the configuration of the DNS settings of the Edge Transport server. Therefore, you can resolve this problem by correcting the DNS configuation.
This is only one times happened in 2 years. So i not sure if this really a DNS configration issue.
The edge server has 2 network cards.
Network1 was label as External Lan has Public IP address (IPv4) and 2 internal DNS assigned.
Network2 was label as Internal Lan has Public IP adddress (IPv4),subnet mask but NO DNS assigned.
The adapter and bindings at "Advanced Settings" was set Network2 at top follow by Network1
The DNS configuration on the Edge Transport server:
Internal DNS Lookups - set to All availablle
External DNS lookups - set to All available.
When we reboot Primary DNS, and do nslookup for internal servers on Edge server, nslookup failed to resolve the internal server. It seems that it doesn't query Secondary DNS server when primary DNS not available.
Does the DNS configuration settings is correct on Edge server?
Why nslookup failed when primary DNS reboot?
It is possibility that the "internal DNS lookups" on Edge look for Network 2 (NO DNS assigned) which cause the issue?