mxmail.register.com has no CNAME records.
Question and response
Question
dig mxmail.register.com. NS
Answer
The response has no answer records.
Authority
register.com. 1705 SOA heidi.ns.cloudflare.com. dns.cloudflare.com. 2396800713 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0