mail.smtp41.com has no NS records.
NS records for this zone are defined at smtp41.com.
NS records for this zone are defined at smtp41.com.
Question and response
Question
dig mail.smtp41.com. NS
Answer
The response has no answer records.
Authority
smtp41.com. 1800 SOA amy.ns.cloudflare.com. dns.cloudflare.com. 2396352545 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0