mx.dts-online.net has no CNAME records.
Question and response
Question
dig mx.dts-online.net. NS
Answer
The response has no answer records.
Authority
dts-online.net. 3262 SOA ns1.dts-online.net. hostmaster.dts-online.net. 2026022302 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0