forward600a.mail.yandex.net has no CNAME records.
Question and response
Question
dig forward600a.mail.yandex.net. NS
Answer
The response has no answer records.
Authority
mail.yandex.net. 300 SOA ns3.yandex.ru. sysadmin.yandex.ru. 1771741251 900 600 3600000 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0