mx.YANDEX.RU has no CNAME records.
Question and response
Question
dig mx.YANDEX.RU. NS
Answer
The response has no answer records.
Authority
YANDEX.RU. 227 SOA ns1.YANDEX.RU. sysadmin.yandex-team.RU. 2023037795 600 300 2592000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0