iris.arpa has no MX records.
Question and response
Question
dig iris.arpa. MX
Answer
The response has no answer records.
Authority
iris.arpa. 3600 SOA sns.dns.icann.org. noc.dns.icann.org. 2022091536 10800 3600 1209600 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0