dns06.salesforce.com has no MX records.
Question and response
Question
dig dns06.salesforce.com. MX
Answer
The response has no answer records.
Authority
salesforce.com. 1800 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2016652580 600 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0