mx.pc-net.ru has no CNAME records.
Question and response
Question
dig mx.pc-net.ru. NS
Answer
The response has no answer records.
Authority
pc-net.ru. 3519 SOA pcs-isp-1.pc-net.ru. root.pc-net.ru. 2021032992 3600 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0