ns01.yahoo.co.jp has no MX records.
Question and response
Question
dig ns01.yahoo.co.jp. MX
Answer
The response has no answer records.
Authority
yahoo.co.jp. 900 SOA yahoo.co.jp. postmaster.yahoo.co.jp. 2602270011 1800 900 86400 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0