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