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. 900 SOA ns1.yandex.ru. sysadmin.yandex-team.ru. 2023037800 600 300 2592000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0