mail.seppmail.io has no CNAME records.
Question and response
Question
dig mail.seppmail.io. NS
Answer
The response has no answer records.
Authority
seppmail.io. 1525 SOA ns.seppmail.io. hostmaster.seppmail.io. 2026030703 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0