spf.mail.knockbot.jp has no MX records.
Question and response
Question
dig spf.mail.knockbot.jp. MX
Answer
The response has no answer records.
Authority
knockbot.jp. 300 SOA 01.dnsv.jp. hostmaster.dnsv.jp. 1756774050 3600 900 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0