spf.acumbamail.com has no MX records.
Question and response
Question
dig spf.acumbamail.com. MX
Answer
The response has no answer records.
Authority
acumbamail.com. 180 SOA ns10.dnsmadeeasy.com. dns.dnsmadeeasy.com. 2009010688 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0