c.in-addr-servers.arpa has no MX records.
Question and response
Question
dig c.in-addr-servers.arpa. MX
Answer
The response has no answer records.
Authority
in-addr-servers.arpa. 3600 SOA b.in-addr-servers.arpa. nstld.iana.org. 2026010762 14400 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0