mx000a.shellrent.com has no CNAME records.
Question and response
Question
dig mx000a.shellrent.com. NS
Answer
The response has no answer records.
Authority
shellrent.com. 701 SOA ada.ns.cloudflare.com. dns.cloudflare.com. 2397011409 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0