spf-a.customercenter.net has no MX records.
Question and response
Question
dig spf-a.customercenter.net. MX
Answer
The response has no answer records.
Authority
customercenter.net. 300 SOA dns2.p07.nsone.net. domainadmin.firstdata.com. 763100760 1200 180 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0