spf.guardedhost.com has no MX records.
Question and response
Question
dig spf.guardedhost.com. MX
Answer
The response has no answer records.
Authority
guardedhost.com. 900 SOA primary.guardeddns.net. dns-admin.guardeddns.net. 347 28800 450 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0