mail.vnslns.com has no CNAME records.
Question and response
Question
dig mail.vnslns.com. NS
Answer
The response has no answer records.
Authority
vnslns.com. 1495 SOA pns31.cloudns.net. support.cloudns.net. 2026022503 7200 1800 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0