smtp.arsystel.com has no CNAME records.
Question and response
Question
dig smtp.arsystel.com. NS
Answer
The response has no answer records.
Authority
arsystel.com. 600 SOA ns-arsys.ui-dns.es. dnsadmin.ionos.com. 2024032519 14400 3600 605000 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0