mail.knownhost.com has no NS records.
NS records for this zone are defined at knownhost.com.
NS records for this zone are defined at knownhost.com.
Question and response
Question
dig mail.knownhost.com. NS
Answer
The response has no answer records.
Authority
knownhost.com. 1800 SOA cns1.knownhost.com. dns.cloudflare.com. 2398336864 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0