mail.plusvps.com has no CNAME records.
Question and response
Question
dig mail.plusvps.com. NS
Answer
The response has no answer records.
Authority
plusvps.com. 1585 SOA ns1.phg.hr. server.plus.hr. 2026030504 1800 3600 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0