181.in-addr.arpa has no MX records.
Question and response
Question
dig 181.in-addr.arpa. MX
Answer
The response has no answer records.
Authority
181.in-addr.arpa. 10800 SOA NS.LACNIC.NET. hostmaster.LACNIC.NET. 2026022517 1800 900 691200 10800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0