mx20.world-serv.net has no NS records.
NS records for this zone are defined at world-serv.net.
NS records for this zone are defined at world-serv.net.
Question and response
Question
dig mx20.world-serv.net. NS
Answer
The response has no answer records.
Authority
world-serv.net. 1800 SOA dan.ns.cloudflare.com. dns.cloudflare.com. 2394563374 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0