mail.donweb.com has no CNAME records.
Question and response
Question
dig mail.donweb.com. NS
Answer
The response has no answer records.
Authority
donweb.com. 1129 SOA carol.ns.cloudflare.com. dns.cloudflare.com. 2396152988 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0