mailservers.supportbee.com has no CNAME records.
Question and response
Question
dig mailservers.supportbee.com. NS
Answer
The response has no answer records.
Authority
supportbee.com. 1552 SOA eva.ns.cloudflare.com. dns.cloudflare.com. 2397293884 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0