121.IN-ADDR.ARPA has no MX records.
Question and response
Question
dig 121.IN-ADDR.ARPA. MX
Answer
The response has no answer records.
Authority
121.in-addr.arpa. 3600 SOA ns.apnic.net. read-txt-record-of-zone-first-dns-admin.apnic.net. 3006102375 7200 1800 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0