smtp01-ar3-sp1.mta.salesforce.com has no MX records.
Question and response
Question
dig smtp01-ar3-sp1.mta.salesforce.com. MX
Answer
The response has no answer records.
Authority
mta.salesforce.com. 600 SOA dns01.salesforce.com. hostmaster.salesforce.com. 2014069076 600 900 2592000 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0