spf.dox.jp has no MX records.
Question and response
Question
dig spf.dox.jp. MX
Answer
The response has no answer records.
Authority
dox.jp. 300 SOA dns-b.iij.ad.jp. dns-managers.iij.ad.jp. 1654849119 3600 1800 3600000 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0