mail01.smtpio.com has no NS records.
NS records for this zone are defined at smtpio.com.
NS records for this zone are defined at smtpio.com.
Question and response
Question
dig mail01.smtpio.com. NS
Answer
The response has no answer records.
Authority
smtpio.com. 1800 SOA isaac.ns.cloudflare.com. dns.cloudflare.com. 2396535168 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0