mail.builder.ru.eserver.net has no CNAME records.
Question and response
Question
dig mail.builder.ru.eserver.net. NS
Answer
The response has no answer records.
Authority
ru.eserver.net. 1800 SOA ns1.ru.eserver.net. hostmaster.eserver.ru. 1769713172 7200 3600 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0