Apex domains cannot have CNAME records.
Question and response
Question
dig gslb.gpphosted.com. NS
Answer
gslb.gpphosted.com. 3 NS ns-ut7.proofpoint.com.
gslb.gpphosted.com. 3 NS ns-sc4.proofpoint.com.
gslb.gpphosted.com. 3 NS ns-at1.proofpoint.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0