dns4.mastercard.com has no MX records.
Question and response
Question
dig dns4.mastercard.com. MX
Answer
The response has no answer records.
Authority
mastercard.com. 300 SOA dns1.mastercard.com. hostmaster.mastercard.com. 2011131199 14400 3600 2419200 300
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0