mx6.sophos.com has no NS records.
NS records for this zone are defined at sophos.com.
NS records for this zone are defined at sophos.com.
Question and response
Question
dig mx6.sophos.com. NS
Answer
The response has no answer records.
Authority
sophos.com. 300 SOA asia1.akam.net. hostmaster.sophos.com. 2020012650 7200 3600 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0