mx.as3257.net has no CNAME records.
Question and response
Question
dig mx.as3257.net. NS
Answer
The response has no answer records.
Authority
as3257.net. 1378 SOA pns.tinet.net. netguard.ip4.tinet.net. 2025081201 86400 7200 3600000 172800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0