mx.yandex.net has no MX records.
Question and response
Question
dig mx.yandex.net. MX
Answer
The response has no answer records.
Authority
yandex.net. 900 SOA ns1.yandex.net. hostmaster.yandex.net. 2023054775 600 300 1209600 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0