mx4.sco.cisco.com has no CNAME records.
Question and response
Question
dig mx4.sco.cisco.com. NS
Answer
The response has no answer records.
Authority
sco.cisco.com. 277 SOA use1.akam.net. stbu-hostmaster.cisco.com. 2026021203 1200 300 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0