239.in-addr.arpa has no MX records.
Question and response
Question
dig 239.in-addr.arpa. MX
Answer
The response has no answer records.
Authority
239.in-addr.arpa. 3600 SOA sns.dns.icann.org. noc.dns.icann.org. 2022091410 7200 3600 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0