mx01.esn.at has no CNAME records.
Question and response
Question
dig mx01.esn.at. NS
Answer
The response has no answer records.
Authority
esn.at. 2870 SOA ns1-38.azure-dns.com. hostmaster.esn.at. 2015072776 3600 900 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0