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