smtp-cluster.plusvps.com has no CNAME records.
Question and response
Question
dig smtp-cluster.plusvps.com. NS
Answer
The response has no answer records.
Authority
plusvps.com. 1758 SOA ns1.phg.hr. server.plus.hr. 2026030504 1800 3600 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0