mail.groupflow.app has no CNAME records.
Question and response
Question
dig mail.groupflow.app. NS
Answer
The response has no answer records.
Authority
groupflow.app. 659 SOA ns1.groupflow.app. hostmaster.groupflow.app. 1 86400 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0