mail.vcosa.com has no CNAME records.
Question and response
Question
dig mail.vcosa.com. NS
Answer
The response has no answer records.
Authority
vcosa.com. 10660 SOA vcosa.com. root.vcosa.com. 2022101001 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0