mx.worldserver.net has no NS records.
NS records for this zone are defined at worldserver.net.
NS records for this zone are defined at worldserver.net.
Question and response
Question
dig mx.worldserver.net. NS
Answer
The response has no answer records.
Authority
worldserver.net. 1800 SOA coen.ns.cloudflare.com. dns.cloudflare.com. 2394910992 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0