192.in-addr.arpa has no MX records.
Question and response
Question
dig 192.in-addr.arpa. MX
Answer
The response has no answer records.
Authority
192.in-addr.arpa. 10800 SOA z.arin.net. dns-ops.arin.net. 2017040154 1800 900 691200 10800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0