ns2.xserver.jp has no MX records.
Question and response
Question
dig ns2.xserver.jp. MX
Answer
The response has no answer records.
Authority
xserver.jp. 3600 SOA ns1.xserver.jp. root.sv0.xserver.jp. 0 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0