forward600b.mail.yandex.net has no CNAME records.
Question and response
Question
dig forward600b.mail.yandex.net. NS
Answer
The response has no answer records.
Authority
mail.yandex.net. 160 SOA ns3.yandex.ru. sysadmin.yandex.ru. 1771743665 900 600 3600000 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0