ipv4only.arpa has no MX records.
Question and response
Question
dig ipv4only.arpa. MX
Answer
The response has no answer records.
Authority
ipv4only.arpa. 3600 SOA sns.dns.icann.org. noc.dns.icann.org. 2022072100 7200 3600 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0