Apex domains cannot have CNAME records.
Question and response
Question
dig proofpoint.com.gslb.pphosted.COM. NS
Answer
proofpoint.com.gslb.pphosted.com. 5 NS ns-sc4.proofpoint.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0