ext.postfix.smedia.tools has no CNAME records.
Question and response
Question
dig ext.postfix.smedia.tools. NS
Answer
The response has no answer records.
Authority
smedia.tools. 18 SOA ns-1.seibert-media-dns.net. root.seibert-media.net. 2016011801 3600 3600 86400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0