mx0.grnet.gr has no CNAME records.
Question and response
Question
dig mx0.grnet.gr. NS
Answer
The response has no answer records.
Authority
grnet.gr. 379 SOA ns-pri.grnet.gr. dnsmgr.grnet.gr. 2026021805 86400 7200 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0