mail.smtp40.com has no NS records.
NS records for this zone are defined at smtp40.com.
NS records for this zone are defined at smtp40.com.
Question and response
Question
dig mail.smtp40.com. NS
Answer
The response has no answer records.
Authority
smtp40.com. 1800 SOA amy.ns.cloudflare.com. dns.cloudflare.com. 2397121617 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0