mxbackup.icewarp.net has no CNAME records.
Question and response
Question
dig mxbackup.icewarp.net. NS
Answer
The response has no answer records.
Authority
icewarp.net. 1800 SOA delilah.ns.cloudflare.com. dns.cloudflare.com. 2395844364 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0