mx03.infusionmail.com has no CNAME records.
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 1232, xrcode 0, version 0, flags 0