mail.grserver.gr has no CNAME records.
Question and response
Question
dig mail.grserver.gr. NS
Answer
The response has no answer records.
Authority
grserver.gr. 590 SOA ns-396.awsdns-49.com. awsdns-hostmaster.amazon.com. 2 7200 900 1209600 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0