mail.host904.slaskdatacenter.com has no CNAME records.
Question and response
Question
dig mail.host904.slaskdatacenter.com. NS
Answer
The response has no answer records.
Authority
host904.slaskdatacenter.com. 1000 SOA ns1.slaskdatacenter.com. hostmaster.host904.slaskdatacenter.com. 2021052300 1000 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0