Apex domains cannot have CNAME records.
Question and response
Question
dig stsmail.ro. NS
Answer
stsmail.ro. 5 NS ns2.stsisp.ro.
stsmail.ro. 5 NS ns1.stsisp.ro.
stsmail.ro. 5 NS ns3.stsisp.ro.
Authority
The response has no authority records.
Additional
ns1.stsisp.ro. 5 A 193.151.29.206
ns2.stsisp.ro. 5 A 193.151.29.246
ns3.stsisp.ro. 5 A 193.151.31.3
ns1.stsisp.ro. 5 AAAA 2a00:5dc2:0:1:0:0:aa:53
ns2.stsisp.ro. 5 AAAA 2a00:5dc2:0:2:0:0:aa:53
ns3.stsisp.ro. 5 AAAA 2a00:5dc2:0:3:0:0:aa:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0