spf.rev.io has no MX records.
Question and response
Question
dig spf.rev.io. MX
Answer
The response has no answer records.
Authority
rev.io. 1800 SOA ns1-03.azure-dns.com. domreg.overgroup.com. 220 3600 600 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0