Apex domains cannot have CNAME records.
Question and response
Question
dig elastx.email. NS
Answer
elastx.email. 5 NS ns31.constellix.com.
elastx.email. 5 NS ns11.constellix.com.
elastx.email. 5 NS ns21.constellix.com.
elastx.email. 5 NS ns61.constellix.net.
elastx.email. 5 NS ns51.constellix.net.
elastx.email. 5 NS ns41.constellix.net.
Authority
The response has no authority records.
Additional
ns11.constellix.COM. 5 A 96.45.80.1
ns21.constellix.COM. 5 A 46.31.236.1
ns31.constellix.COM. 5 A 43.247.170.1
ns11.constellix.COM. 5 AAAA 2600:180a:1001:0:0:0:0:1
ns21.constellix.COM. 5 AAAA 2600:180b:2001:0:0:0:0:1
ns31.constellix.COM. 5 AAAA 2600:180c:3001:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0