If you are using Microsoft for your DNS servers you really should update to
a version that includes this fix from March 19, 2019 monthly updates.
• Addresses minor issues with unknown options (unknown OPT) in the
Extension Mechanisms for DNS (EDNS) for the Windows DNS Server role.
Older versions have even more egregious misbehaviour, with really old versions
(pre-EDNS support) being flagged as dead by recursive servers as they do not
respond consistently to EDNS queries. They respond a single time then do not
respond at all for a period of time.