spf.mailcore.net has no MX records.
Question and response
Question
dig spf.mailcore.net. MX
Answer
The response has no answer records.
Authority
mailcore.net. 3600 SOA ns21.cloudns.net. postmaster.mailcore.com. 2026021806 7200 1800 1209600 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0