mail.oxismtp.com has no CNAME records.
Question and response
Question
dig mail.oxismtp.com. NS
Answer
The response has no answer records.
Authority
oxismtp.com. 10800 SOA ns.oxismtp.com. postmaster.oxismtp.com. 2025040402 10800 3600 1209600 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0