smtp.sfera.net has no CNAME records.
Question and response
Question
dig smtp.sfera.net. NS
Answer
The response has no answer records.
Authority
sfera.net. 271 SOA dns.sfera.net. hostmaster.dns.sfera.net. 2019033298 3600 1800 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0