external.mastercard.com has no CNAME records.
Question and response
Question
dig external.mastercard.com. NS
Answer
The response has no answer records.
Authority
mastercard.com. 249 SOA dns1.mastercard.com. hostmaster.mastercard.com. 2011131199 14400 3600 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0