smail.ucloudbiz.com has no CNAME records.
Question and response
Question
dig smail.ucloudbiz.com. NS
Answer
The response has no answer records.
Authority
ucloudbiz.com. 1196 SOA ns.kornet.net. domain.ns.kornet.net. 2022111500 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0