ptr.5x00.com has no CNAME records.
Question and response
Question
dig ptr.5x00.com. NS
Answer
The response has no answer records.
Authority
5x00.com. 3284 SOA reutov.ens.mail.ru. noreply.biz.mail.ru. 2022120507 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0