mailer01.SEND-BOX.RU has no CNAME records.
Question and response
Question
dig mailer01.SEND-BOX.RU. NS
Answer
The response has no answer records.
Authority
SEND-BOX.RU. 300 SOA ns1.mail.RU. hostmaster.mail.RU. 3491252382 3600 900 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0