mta.mktgc.salesforce.com has no MX records.
Question and response
Question
dig mta.mktgc.salesforce.com. MX
Answer
The response has no answer records.
Authority
mktgc.salesforce.com. 3600 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1719604012 7200 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0