ns1.softbankbb.jp has no MX records.
Question and response
Question
dig ns1.softbankbb.jp. MX
Answer
The response has no answer records.
Authority
softbankbb.jp. 3600 SOA ns1.softbankbb.jp. dns-info.bb.softbank.co.jp. 2013031101 3600 1800 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0