mail000.shellrent.com has no CNAME records.
Question and response
Question
dig mail000.shellrent.com. NS
Answer
The response has no answer records.
Authority
shellrent.com. 1800 SOA ada.ns.cloudflare.com. dns.cloudflare.com. 2397321739 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0