spf.dnssmarthost.net has no MX records.
Question and response
Question
dig spf.dnssmarthost.net. MX
Answer
The response has no answer records.
Authority
spf.dnssmarthost.net. 86400 SOA ns1.spf.dnssmarthost.net. admins.siteground.com. 9004448 86400 7200 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0