mail.isgroup.info has no NS records.
NS records for this zone are defined at isgroup.info.
NS records for this zone are defined at isgroup.info.
Question and response
Question
dig mail.isgroup.info. NS
Answer
The response has no answer records.
Authority
isgroup.info. 1800 SOA newt.ns.cloudflare.com. dns.cloudflare.com. 2398211067 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0