mx03.infusionmail.com has no NS records.
NS records for this zone are defined at infusionmail.com.
NS records for this zone are defined at infusionmail.com.
Question and response
Question
dig mx03.infusionmail.com. NS
Answer
The response has no answer records.
Authority
infusionmail.com. 300 SOA ns-cloud-d1.googledomains.com. support.infusionsoft.com. 2019110701 3600 180 1209600 3600
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0