mail4.smtp12.com has no NS records.
NS records for this zone are defined at smtp12.com.
NS records for this zone are defined at smtp12.com.
Question and response
Question
dig mail4.smtp12.com. NS
Answer
The response has no answer records.
Authority
smtp12.com. 1800 SOA amy.ns.cloudflare.com. dns.cloudflare.com. 2395382460 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0