spf310.gcb.su has no CNAME records.
Question and response
Question
dig spf310.gcb.su. NS
Answer
The response has no answer records.
Authority
gcb.su. 2539 SOA ns-mail03.getcourse.ru. hostmaster.gcb.su. 672197 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0