mta.salesforce.com has no MX records.
Question and response
Question
dig mta.salesforce.com. MX
Answer
The response has no answer records.
Authority
mta.salesforce.com. 600 SOA dns01.salesforce.com. hostmaster.salesforce.com. 2014069070 600 900 2592000 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0