mx.zsttk.ru has no CNAME records.
Question and response
Question
dig mx.zsttk.ru. NS
Answer
The response has no answer records.
Authority
zsttk.ru. 120 SOA ns.zsttk.ru. hostmaster.zsttk.ru. 2020061617 60 30 604800 120
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0