ns1.cisco.com has no MX records.
Question and response
Question
dig ns1.cisco.com. MX
Answer
The response has no answer records.
Authority
cisco.com. 1800 SOA ns1.cisco.com. postmaster.cisco.com. 1177806 7200 1800 864000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0