app.testing.vinmail.io has no CNAME records.
Question and response
Question
dig app.testing.vinmail.io. NS
Answer
The response has no answer records.
Authority
vinmail.io. 1679 SOA aron.ns.cloudflare.com. dns.cloudflare.com. 2396903644 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0