133.in-addr.arpa has no MX records.
Question and response
Question
dig 133.in-addr.arpa. MX
Answer
The response has no answer records.
Authority
133.in-addr.arpa. 86400 SOA y.dns.jp. root.dns.jp. 2026031001 3600 900 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0