51.121.27.in-addr.arpa has no MX records.
Question and response
Question
dig 51.121.27.in-addr.arpa. MX
Answer
The response has no answer records.
Authority
51.121.27.in-addr.arpa. 900 SOA dns1.broadline.ne.jp. dns-info.broadline.ne.jp. 45 10800 900 604800 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0