mxold.mail-zone.net has no CNAME records.
Question and response
Question
dig mxold.mail-zone.net. NS
Answer
The response has no answer records.
Authority
mail-zone.net. 632 SOA kiki.bunny.net. hostmaster.bunny.net. 2021325109 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0