_spfblock.salesforce.com has no MX records.
Question and response
Question
dig _spfblock.salesforce.com. MX
Answer
The response has no answer records.
Authority
salesforce.com. 1800 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2016652495 600 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0