Apex domains cannot have CNAME records.
Question and response
Question
dig mailserv.se. NS
Answer
mailserv.se. 1 NS ns1.dnsserv.nu.
mailserv.se. 1 NS ns1.dnsserv.eu.
mailserv.se. 1 NS ns1.dnsserv.name.
mailserv.se. 1 NS ns1.dnsserv.uk.
mailserv.se. 1 NS ns1.dnsserv.se.
Authority
The response has no authority records.
Additional
ns1.dnsserv.eu. 1 A 194.58.198.142
ns1.dnsserv.nu. 1 A 31.22.94.70
ns1.dnsserv.se. 1 A 193.104.214.170
ns1.dnsserv.uk. 1 A 194.58.193.62
ns1.dnsserv.name. 1 A 185.42.137.119
ns1.dnsserv.eu. 1 AAAA 2a01:3f1:3066:0:0:0:0:53
ns1.dnsserv.nu. 1 AAAA 2a0c:5700:5353:0:0:0:0:70
ns1.dnsserv.se. 1 AAAA 2001:67c:25a4:c64:242:53:53:53
ns1.dnsserv.uk. 1 AAAA 2a01:3f1:462:0:0:0:0:53
ns1.dnsserv.name. 1 AAAA 2a01:3f0:400:0:0:0:0:66
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0