mx.ipv6.zsttk.net has no CNAME records.
Question and response
Question
dig mx.ipv6.zsttk.net. NS
Answer
The response has no answer records.
Authority
ipv6.zsttk.net. 10800 SOA ns.zsttk.ru. hostmaster.zsttk.ru. 2018082100 43200 3600 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0