salesforce-dns.com has no MX records.
Question and response
Question
dig salesforce-dns.com. MX
Answer
The response has no answer records.
Authority
salesforce-dns.com. 1800 SOA dns01.salesforce.com. hostmaster.salesforce.com. 2018081087 1800 900 2592000 1800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0