spf.stackmail.com has no MX records.
Question and response
Question
dig spf.stackmail.com. MX
Answer
The response has no answer records.
Authority
stackmail.com. 300 SOA ns1.stackdns.com. hostmaster.stackdns.com. 1666943089 1800 900 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0