mail.worldserver.net has no CNAME records.
Question and response
Question
dig mail.worldserver.net. NS
Answer
The response has no answer records.
Authority
worldserver.net. 1446 SOA coen.ns.cloudflare.com. dns.cloudflare.com. 2394910992 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0