cron01.mxguardian.net has no NS records.
NS records for this zone are defined at mxguardian.net.
NS records for this zone are defined at mxguardian.net.
Question and response
Question
dig cron01.mxguardian.net. NS
Answer
The response has no answer records.
Authority
mxguardian.net. 3600 SOA ns01.mxguardian.net. support.mxguardian.net. 1773583075 3600 1800 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0