Apex domains cannot have CNAME records.
Question and response
Question
dig proofpoint.us. NS
Answer
proofpoint.us. 5 NS pdns99.ultradns.com.
proofpoint.us. 5 NS pdns99.ultradns.net.
proofpoint.us. 5 NS ns1.proofpoint.us.
proofpoint.us. 5 NS pdns99.ultradns.biz.
proofpoint.us. 5 NS pdns99.ultradns.org.
proofpoint.us. 5 NS ns3.proofpoint.us.
Authority
The response has no authority records.
Additional
ns1.proofpoint.us. 5 A 208.84.67.208
ns3.proofpoint.us. 5 A 208.84.66.208
ns1.proofpoint.us. 5 AAAA 2620:100:9000:1:0:0:0:d0
ns3.proofpoint.us. 5 AAAA 2620:100:9004:1:0:0:0:d0
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0