RIRNS.ARIN.NET has no MX records.
Question and response
Question
dig RIRNS.ARIN.NET. MX
Answer
The response has no answer records.
Authority
arin.net. 3600 SOA ns1.arin.net. bind.arin.net. 2021113220 10800 600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0