arpa has no MX records.
Question and response
Question
dig arpa. MX
Answer
The response has no answer records.
Authority
arpa. 86400 SOA a.root-servers.net. nstld.verisign-grs.com. 2026022701 1800 900 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0