mx.zeptomail.com has no CNAME records.
Question and response
Question
dig mx.zeptomail.com. NS
Answer
The response has no answer records.
Authority
zeptomail.com. 600 SOA ns1.zohocorp.com. dnsadmin.vtitan.com. 2026022203 7200 1800 1209600 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0