mail.hostingnetwork.at has no CNAME records.
Question and response
Question
dig mail.hostingnetwork.at. NS
Answer
The response has no answer records.
Authority
hostingnetwork.at. 5795 SOA ns1.internex.at. info.internex.at. 2013080900 43200 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0