mail.grserver.gr has no NS records.
NS records for this zone are defined at grserver.gr.
NS records for this zone are defined at grserver.gr.
Question and response
Question
dig mail.grserver.gr. NS
Answer
The response has no answer records.
Authority
grserver.gr. 900 SOA ns-396.awsdns-49.com. awsdns-hostmaster.amazon.com. 2 7200 900 1209600 1800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0