smtp-corp.mar.freepro.com has no CNAME records.
Question and response
Question
dig smtp-corp.mar.freepro.com. NS
Answer
The response has no answer records.
Authority
freepro.com. 10800 SOA ns1.as30781.net. hostmaster.as30781.net. 2026030501 21600 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0