e164.arpa has no MX records.
Question and response
Question
dig e164.arpa. MX
Answer
The response has no answer records.
Authority
e164.arpa. 3600 SOA pri.authdns.ripe.net. dns.ripe.net. 1772113170 3600 600 864000 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0