mx.apronix.net has no CNAME records.
Question and response
Question
dig mx.apronix.net. NS
Answer
The response has no answer records.
Authority
apronix.net. 3510 SOA dns1w.brunner.at. hostmaster.apronix.net. 201902062 3600 7200 86400 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0