spf.amnic.net has no MX records.
Question and response
Question
dig spf.amnic.net. MX
Answer
The response has no answer records.
Authority
amnic.net. 14400 SOA ns-pri.amnic.net. dns.amnic.net. 1770637756 43200 7200 1209600 14400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0