Apex domains cannot have CNAME records.
Question and response
Question
dig post.at. NS
Answer
post.at. 5 NS dns3.a1.net.
post.at. 5 NS dns2.a1.net.
post.at. 5 NS dns1.a1.net.
Authority
The response has no authority records.
Additional
dns1.a1.net. 5 A 80.120.17.26
dns2.a1.net. 5 A 192.174.68.105
dns2.a1.net. 5 A 213.33.99.79
dns3.a1.net. 5 A 176.97.158.105
dns1.a1.net. 5 AAAA 2001:850:3030:31:80:120:17:26
dns2.a1.net. 5 AAAA 2001:67c:1bc:0:0:0:0:105
dns2.a1.net. 5 AAAA 2001:850:3010:101:213:33:99:79
dns3.a1.net. 5 AAAA 2001:67c:10b8:0:0:0:0:105
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0