ip6.arpa has no MX records.
Question and response
Question
dig ip6.arpa. MX
Answer
The response has no answer records.
Authority
ip6.arpa. 3600 SOA b.ip6-servers.arpa. nstld.iana.org. 2026021945 1800 900 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0