mail.secstone.at has no CNAME records.
Question and response
Question
dig mail.secstone.at. NS
Answer
The response has no answer records.
Authority
secstone.at. 10800 SOA root-dns.netcup.net. dnsadmin.netcup.net. 2026021586 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0